Forretress's ability to switch into so many threats and provide entry hazards and Rapid Spin support is unmatched. While it may have many obvious flaws, such as its limited ability to defend itself and its horrible Fire weakness, these issues are easily addressed by the rest of the team. One looking for a reliable entry hazard abuser and Rapid Spinner that can take an onslaught of physical attacks won't find anything better than Forretress.
This is your classic bread and butter Forretress set. The general idea is to switch in when Forretress is not threatened, and take the opportunity to support the team with entry hazards or Rapid Spin enemy hazards away. With its massive Defense stat, huge list of resistances, and the general abundance of resisted attacks being used with Choice items in today's metagame, there should be plenty of opportunities to execute this strategy.
Forretress has a variety of attacking options available to it. Gyro Ball takes advantage of its abysmal Speed to provide it with a powerful STAB attack that deals solid damage to fast attackers, such as Gengar, Dragon Dance Dragonite, and Choice Scarf or Dragon Dance Tyranitar. Payback is useful for hitting Ghosts that switch in to block Forretress's Rapid Spin, and because Forretress is so slow, it will almost always be 100 Base Power. Earthquake is a good option to cover grounded Steel-types like Metagross, Lucario, and Jirachi. In addition, Forretress can use Earthquake to land OHKOs on many of its counters as they switch into it, specifically Magnezone, Heatran, and Infernape. As Forretress has a tendency to become setup fodder for many sweepers that aren't threatened by its other attacks, Explosion is a fine choice on this set to immediately end a powerful, potentially game-ending sweep from your opponent. On an offensive team, it can be used to eliminate a defensive check against one of your sweepers. As on any Pokemon, the combination of attacks you choose to run should complement the needs of your team as a whole.Team Options & Additional Comments >>>
Although its most prominent stat is its massive Defense, Forretress's advantageous resistances to common special attacks including Ice Beam, Shadow Ball, and Draco Meteor make it a great response to a variety of special threats in the metagame. With a specially defensive EV spread and a Careful nature, Forretress becomes much more capable of taking powerful special attacks like Dragonite's Draco Meteor and Gengar's Focus Blast, while still retaining enough physical bulk to switch into and set up on physical threats whose STABs it resists, such as Metagross, Scizor, and Jirachi. Because of Forretress's abysmal Speed, Payback becomes its most powerful choice on this set. At 100 Base Power, it makes for a strong, super effective attack against Gengar, Rotom-A, and Celebi, all of which are incapable of dispatching Forretress with any of their attacks, bar Hidden Power Fire.Team Options & Additional Comments >>>
Forretress doesn't have many moves available to it other than those listed in the sets, though it does have a few mildly interesting choices. Revenge is another attack that takes advantage of Forretress's low Speed, providing coverage against Lucario, Tyranitar and, to an extent, Magnet Rise Magnezone. Reflect and Light Screen are decent options on Forretress, both for supporting its team and making it easier for Forretress to set up hazards against many hard-hitting threats. RestTalk gives Forretress a somewhat reliable form of recovery and allow it to become a status absorber. However, this will further limit the amount of team support it can provide, and there are much better options for status absorbers available. Pain Split is another recovery option, although Forretress is already packed for space on its support sets and Pain Split isn't too reliable anyway.
For Forretress sets with Gyro Ball, Macho Brace makes an interesting alternative to Leftovers or Shed Shell, as it lowers Forretress's Speed stat further. Assuming a 0 Speed IV and a negative Speed nature, Macho Brace turns Gyro Ball into a 100 Base Power attack against anything with 198 Speed and above, and a 150 Base Power attack against anything above 297 Speed. This will allow Forretress to be more of a threat to Dragonite, and Gyro Ball will be capable of OHKOing Gengar at full health, something it can't do without Macho Brace.
As a defensive Pokemon, the obvious first step is to maximize Forretress's HP. From there, you can decide how you want to split the EVs between its respective defenses and its Attack to suit your team's needs. The Physically Defensive EV spread allows Forretress to OHKO 4 HP / 0 Defense Magnezone with Earthquake and Stealth Rock, but for sets without Earthquake, one may wish to use a bulkier EV spread. A set using Gyro Ball should have a Speed IV of 0 and a –Speed nature to maximize power output. The specially defensive EV spread really doesn't need any more Attack EVs to ensure the 3HKO on standard Rotom-A in sandstorm, and Forretress needs as much SpD as possible.
Checks and Counters
Heatran and Magnezone can switch into any one of Forretress's moves, barring Earthquake, and threaten to OHKO with Fire Blast and Hidden Power Fire, respectively. Magnezone's ability, Magnet Pull, will also keep non-Shed Shell Forretress from switching away from it. Zapdos is virtually immune to all of Forretress's attacks other than Explosion and can batter it with Thunderbolt or dispatch it in one hit with Heat Wave. The Rotom appliances work similarly to Zapdos, though they must be a bit wary of Payback. However, Rotom has an advantage over other switch-ins in that it blocks Forretress's Rapid Spin due to its Ghost typing. Other Ghost-types, such as Gengar, Dusknoir, and Spiritomb can be used for a similar purpose, though they should have a specific way to defeat Forretress, whether it be with Hidden Power Fire or by burning it with Will-O-Wisp.
Plenty of Pokemon can set up on Forretress with little to no risk. Gyarados can switch into a Forretress without Explosion or Zap Cannon and boost its stats with Dragon Dance. Kingdra can also set up with Dragon Dance, and it can avoid Explosion through the use of Substitute. If Forretress is without Earthquake, Lucario can set up with Swords Dance and threaten to sweep through the opposing team. Bulky Calm Minders and Cursers, such as Suicune, Jirachi, Snorlax, and Swampert, can switch into Forretress with relative ease and begin to set up with their respective stat up moves.
Although Forretress technically has a lot of counters, it thrives off of free turns upon which it can Rapid Spin or set up entry hazards. Therefore, it's important to take Forretress seriously and respond to it immediately, as depending on the opposing team, it may take only a few turns for it to set up several layers of hazards and solidify a win. It's also a good idea to limit the number of Pokemon on your team that are vulnerable to hazards, particularly Toxic Spikes. If one is desperate to lure out and eliminate Forretress, a good strategy is to use a Pokemon which Forretress normally switches into and equip it with a Fire-type attack to strike its 4x weakness. Fire Punch Jirachi, Flamethrower Blissey, and Hidden Power Fire Metagross are all good examples of this.