http://www.smogon.com/dp/pokemon/forretress
Not really a Platinum Update, but I felt that the analysis was a bit out-dated and a little under-written, so I did these:
• Removed Garchomp reference
• I am voting to remove the Light Screen set, no-one uses it (Doug’s Stats show this) and in all honesty you’re better off just running away with Shed Shell and coming back later to continue set up. It is a huge gimmick that only seems to be there to fill out the analysis a bit more, I will drop it to Other Options for now.
• Made Shed Shell the primary option on the set
• Put Stealth Rock in Set Comments rather than in Other Options as apparently 44% of Forretress use it
• Evs have been changed to 252 HP / 112 Atk / 144 Def – You still OHKO Weavile with Gyro Ball, and you still OHKO 4 HP / 0 Spe (Scarf) Magnezone with Earthquake after Stealth Rock. I don’t think OHKOing Adamant 252 variants of Tyranitar after a Dragon Dance is really a priority, unless you can convince me ;)
• Re-wrote first set's comments, they seemed a little "thin"
• Forretress is less commonly seen outside of stall teams so I wrote the analysis more focused on that aspect, as is most noticeable in “Opinion”
• I left EVs alone, Darkie has already dealt with that, same goes for Counters.
[SET]
name: Standard
move 1: Toxic Spikes / Spikes
move 2: Gyro Ball / Earthquake
move 3: Rapid Spin
move 4: Explosion / Reflect / Rest
item: Shed Shell / Leftovers
nature: Relaxed
evs: 252 HP / 112 Atk / 144 Def
ivs: 0 Spe
[SET COMMENTS]
<p>This is by far Forretress’ most useful set. Forretress is one of the metagame’s premier Spikers, due to its large Defense stat and huge number of resistances. It is a major player in a lot of successful stall teams. The idea is simply to come in where Forretress isn’t threatened and set up. These situations are surprisingly common. For example: it can come in on any Weavile set, Choice Band Tyranitar stuck on Crunch, and even a Salamence locked into Outrage!</p>
<p>The hazards that you use depend on what you want your team to accomplish; Spikes punish those teams that like to switch around a lot, and Toxic Spikes will ruin Pokemon that like to stay in for a long time such as Hippowdon or Swampert. Toxic Spikes comes with the disadvantage of being absorbed by other Poison-types, and both lose to Rapid Spinners, so Spin blocking support is necessary to use this set to its greatest effect. Stealth Rock may also be used on this set, but there are fewer viable Spikers than there are Stealth Rock users, so it is more practical to find somewhere else on your team for it. Gyro Ball allows you to deal hard damage to Weavile and Tyranitar, especially Dragon Dance variants, but Earthquake will allow you to hit Magnezone and Poison-types who might want to absorb Toxic Spikes. Rapid Spin will remove opposing team’s entry hazards, but can be removed to accommodate two hazards or two attacks should you have the support elsewhere. Explosion will allow you to deal a lot of damage to something on your opponents’ team, but Rest may be used to allow Forretress to stick around a little longer, especially if you have the support of a cleric. Reflect can give your team a little extra support on the physical side, and helps Forretress survive repeated Stone Edges from Choice Band Tyranitar and Rhyperior.</p>
<p>Shed Shell is the preferred item, as it will allow you to escape from Magnezone, Forretress’ greatest nemesis. Leftovers should only be used if you have reason not to fear Magnezone. Relaxed nature and a 0 Speed IV maximize the power output from Gyro Ball, which proves very useful against faster set-up sweepers. Along with the given EVs, it can OHKO Weavile or, should you use Earthquake, OHKO 4 HP / 0 Def Magnezone on the switch with Stealth Rock support. The rest of the EVs are placed in HP and Defense to allow Forretress to have an easier time setting up and taking hits.</p>
[Other Options]
<p>Giving Forretress Macho Brace allows it to hit anything 198 Speed and above with at least a 100 BP attack, and 150 BP to anything above 297 Speed. Without the Macho Brace, it’s only 66 and 100 BP, respectively, without factoring in STAB. This more powerful Gyro Ball may give Forretress the extra power you might feel it needs to hit foes such as Intimidating Salamence harder. Note also that the Macho Brace can make the difference between OHKOing and not OHKOing Gengar.</p>
<p>Since Forretress is slow; it’s also a good user of Revenge and Payback—although this begs the question of what you would be using them against. Some things that come to mind include Gengar, to whom Payback might OHKO so long as Forretress moves last, and Tyranitar, who you can hit with Revenge. Revenge also covers Magnet Rise Magnezone somewhat.</p>
<p>Zap Cannon allows Forretress to cripple Gengar on the switch, and do some damage to Skarmory—it was not uncommon to see Zap Cannon on a Forretress in Advance. Zap Cannon also allows Forretress to counter Gyarados, although somewhat unreliably. Hidden Power Electric is a better option on Forretress if you want to counter Gyarados. With a neutral nature, a Hidden Power Electric from Forretress can 2HKO Gyarados, taking Stealth Rock into account.</p>
<p>With Light Screen and an EV spread of 252 HP / 96 Def / 162 SpD, an unboosted Magnezone Thunderbolt can only 3HKO you, allowing you to set up 2 layers of Spikes as it tries to trap you, or to 2HKO it with Earthquake. Rest and Sleep Talk can be used in tandem to allow Forretress to set up and heal repeatedly.</p>
[Opinion]
<p>Forretress’ true home is in a stall team, and it does its job very well. It can set up hazards, and then later in the game it can be used to force switches, racking up residual damage and continuing the aim of the team. Forretress is also occasionally seen on more balanced teams based around entry hazards, as it also has the ability to counter a number of dangerous threats. Using it only to counter threats is unwise; the majority of the time it is preferable to use a bulky Scizor set up, as Forretress’ poor attacking type coverage and low Attack stat mean that it can be set up on quite easily.</p>
<p>Forretress is a top tier OU Pokemon because it is good at the job that it does, providing great team support. Do not attempt to do anything fancy with it, odds are it is not cut out for it!</p>

Not really a Platinum Update, but I felt that the analysis was a bit out-dated and a little under-written, so I did these:
• Removed Garchomp reference
• I am voting to remove the Light Screen set, no-one uses it (Doug’s Stats show this) and in all honesty you’re better off just running away with Shed Shell and coming back later to continue set up. It is a huge gimmick that only seems to be there to fill out the analysis a bit more, I will drop it to Other Options for now.
• Made Shed Shell the primary option on the set
• Put Stealth Rock in Set Comments rather than in Other Options as apparently 44% of Forretress use it
• Evs have been changed to 252 HP / 112 Atk / 144 Def – You still OHKO Weavile with Gyro Ball, and you still OHKO 4 HP / 0 Spe (Scarf) Magnezone with Earthquake after Stealth Rock. I don’t think OHKOing Adamant 252 variants of Tyranitar after a Dragon Dance is really a priority, unless you can convince me ;)
• Re-wrote first set's comments, they seemed a little "thin"
• Forretress is less commonly seen outside of stall teams so I wrote the analysis more focused on that aspect, as is most noticeable in “Opinion”
• I left EVs alone, Darkie has already dealt with that, same goes for Counters.
[SET]
name: Standard
move 1: Toxic Spikes / Spikes
move 2: Gyro Ball / Earthquake
move 3: Rapid Spin
move 4: Explosion / Reflect / Rest
item: Shed Shell / Leftovers
nature: Relaxed
evs: 252 HP / 112 Atk / 144 Def
ivs: 0 Spe
[SET COMMENTS]
<p>This is by far Forretress’ most useful set. Forretress is one of the metagame’s premier Spikers, due to its large Defense stat and huge number of resistances. It is a major player in a lot of successful stall teams. The idea is simply to come in where Forretress isn’t threatened and set up. These situations are surprisingly common. For example: it can come in on any Weavile set, Choice Band Tyranitar stuck on Crunch, and even a Salamence locked into Outrage!</p>
<p>The hazards that you use depend on what you want your team to accomplish; Spikes punish those teams that like to switch around a lot, and Toxic Spikes will ruin Pokemon that like to stay in for a long time such as Hippowdon or Swampert. Toxic Spikes comes with the disadvantage of being absorbed by other Poison-types, and both lose to Rapid Spinners, so Spin blocking support is necessary to use this set to its greatest effect. Stealth Rock may also be used on this set, but there are fewer viable Spikers than there are Stealth Rock users, so it is more practical to find somewhere else on your team for it. Gyro Ball allows you to deal hard damage to Weavile and Tyranitar, especially Dragon Dance variants, but Earthquake will allow you to hit Magnezone and Poison-types who might want to absorb Toxic Spikes. Rapid Spin will remove opposing team’s entry hazards, but can be removed to accommodate two hazards or two attacks should you have the support elsewhere. Explosion will allow you to deal a lot of damage to something on your opponents’ team, but Rest may be used to allow Forretress to stick around a little longer, especially if you have the support of a cleric. Reflect can give your team a little extra support on the physical side, and helps Forretress survive repeated Stone Edges from Choice Band Tyranitar and Rhyperior.</p>
<p>Shed Shell is the preferred item, as it will allow you to escape from Magnezone, Forretress’ greatest nemesis. Leftovers should only be used if you have reason not to fear Magnezone. Relaxed nature and a 0 Speed IV maximize the power output from Gyro Ball, which proves very useful against faster set-up sweepers. Along with the given EVs, it can OHKO Weavile or, should you use Earthquake, OHKO 4 HP / 0 Def Magnezone on the switch with Stealth Rock support. The rest of the EVs are placed in HP and Defense to allow Forretress to have an easier time setting up and taking hits.</p>
[Other Options]
<p>Giving Forretress Macho Brace allows it to hit anything 198 Speed and above with at least a 100 BP attack, and 150 BP to anything above 297 Speed. Without the Macho Brace, it’s only 66 and 100 BP, respectively, without factoring in STAB. This more powerful Gyro Ball may give Forretress the extra power you might feel it needs to hit foes such as Intimidating Salamence harder. Note also that the Macho Brace can make the difference between OHKOing and not OHKOing Gengar.</p>
<p>Since Forretress is slow; it’s also a good user of Revenge and Payback—although this begs the question of what you would be using them against. Some things that come to mind include Gengar, to whom Payback might OHKO so long as Forretress moves last, and Tyranitar, who you can hit with Revenge. Revenge also covers Magnet Rise Magnezone somewhat.</p>
<p>Zap Cannon allows Forretress to cripple Gengar on the switch, and do some damage to Skarmory—it was not uncommon to see Zap Cannon on a Forretress in Advance. Zap Cannon also allows Forretress to counter Gyarados, although somewhat unreliably. Hidden Power Electric is a better option on Forretress if you want to counter Gyarados. With a neutral nature, a Hidden Power Electric from Forretress can 2HKO Gyarados, taking Stealth Rock into account.</p>
<p>With Light Screen and an EV spread of 252 HP / 96 Def / 162 SpD, an unboosted Magnezone Thunderbolt can only 3HKO you, allowing you to set up 2 layers of Spikes as it tries to trap you, or to 2HKO it with Earthquake. Rest and Sleep Talk can be used in tandem to allow Forretress to set up and heal repeatedly.</p>
[Opinion]
<p>Forretress’ true home is in a stall team, and it does its job very well. It can set up hazards, and then later in the game it can be used to force switches, racking up residual damage and continuing the aim of the team. Forretress is also occasionally seen on more balanced teams based around entry hazards, as it also has the ability to counter a number of dangerous threats. Using it only to counter threats is unwise; the majority of the time it is preferable to use a bulky Scizor set up, as Forretress’ poor attacking type coverage and low Attack stat mean that it can be set up on quite easily.</p>
<p>Forretress is a top tier OU Pokemon because it is good at the job that it does, providing great team support. Do not attempt to do anything fancy with it, odds are it is not cut out for it!</p>