Forretress
Type Tier
Bug / Steel OU
Statistics
Stat
HP
75
 353
Atk
90
 278
Def
140
 378
SpA
60
 218
SpD
60
 218
Spe
40
 178
Name Item

Spiker

 Leftovers
Moveset
~ Spikes
~ Explosion
~ Rapid Spin
~ Toxic / Reflect / Hidden Power Bug

Spikes is probably the only reason you'd use Forretress, so get those down. Hidden Power Bug can make Forretress a decent counter for Exeggutor and Celebi. Explosion is pretty obvious. Rapid Spin has become almost standard on Forretress with the abundance of Spikes. Much like Blissey being a great Light Screener, Forretress makes a good Reflector as well considering it has a decent amount of resistances and good Defense. Toxic is also an option, albeit a soft one. Forretress is one of the few things that can really use Toxic because that's really the only way it's going to deal any damage to stuff like Zapdos without using Explosion. The prevalence of Rest puts a real damper on any chance of whittling down their health with Toxic, however.

Other Options

Protect can scout for certain moves since you never know what's packing Fire Blast these days, and Substitute can make sure you safely get Spikes down and maybe even get an attack in.

Curse boosts Hidden Power Bug, if you decide to use both. Curse can also be paired with Explosion to deter Mean Look + Perish Song ("Perish Trapping"). Switch in on the Mean Look, set up your Spikes, and start Cursing. On the last turn of Perish Song, Explode on whatever they bring out. This obviously won't work if they have Gengar and Misdreavus, and won't do much if they switch to a Rock or Steel-type, but it's better than nothing.

And of course, since this is GSC, there is always Rest.

Opinion

Forretress is no Skarmory, but it is decent for a physical wall. Forretress is probably the worst Spiker and Rapid Spinner because the three major Rapid Spinners (Cloyster, Starmie, and Donphan) can all beat it (if Starmie has Reflect, Starmie can beat Forretress), and Forretress has nothing for Gengar—but that is why you have 5 other Pokémon on your team! Nonetheless, learning Spikes and having good Defense automatically makes Forretress a good Pokémon, and it can be used well.

Counters

Anything that can take a Hidden Power Bug, if Forretress is even packing that. Most special attackers will be able to put a nice dent in it. You just need to keep Rapid Spinning to lure him out, then nail it with a special move (preferably Fire). Forretress won't last that long.