Forretress @ Leftovers / Light Clay
Relaxed - 252 HP, 184 Atk, 72 Def
Sturdy
~ Stealth Rock
~ Explosion
~ Gyro Ball
~ Reflect / Earthquake / Rapid Spin
This is my favorite set for Forretress. It has great endurance abilities when facing a physical threat. As soon as you fear a special attack coming, just switch out to Blissey or something else.
Reflect greatly helps your team, especially if you are indeed switching to a special wall after you've used it, because that way, the special wall will be able to take physical hits fairly well too, which in Blissey's case is extremely important. Stealth Rock is a better choice than Spikes on this thing, as you won't have as much time to set up with it as with Skarmory.
I chose a Relaxed nature, because you're already slower than most things, so why not drop your Speed to the absolute minimum to take the most out of Gyro Ball (aim for 0 IVs in Speed). STABed Gyro Ball from such a slow pokemon, with 262 Atk definitely hurts a couple of things. Explosion is in for a last resort move, and it helps you set up for your Special Sweeper. With this much Atk EVs, it can OHKO Blissey.
You can consider Rapid Spin on him. If so, I'd replace either Reflect or Earthquake with it, most likely Reflect. As such, it's only logical you'd also pick Leftovers as your hold item, or the Macho Brace if you want to hit pokemon even harder with Gyro Ball, as it halves your Speed.
Forretress @ Leftovers
Relaxed ? 244 HP, 140 SpDef, 124 Def
Sturdy
~ Stealth Rock
~ Light Screen
~ Gyro Ball
~ Explosion
The set is somewhat the same as the first one, only this one is aimed to be a mixed wall. It?s probably not the best thing to do with Forretress, given its low SpDef, but Light Screen can make up for that. With this spread, its HP will be 352, the highest possible Leftovers number it can reach (and only 2 lower than its max HP) and after a Light Screen, its SpDef will be approximately the same as its Def. Its Def is 381 and its SpDef 191, so it?s a 1-point difference after applying the boost Light Screen provides you with. The rest of the set is basically the same, except for the fact that this doesn?t run any Atk EVs. I?d recommend to try and reach 243 Atk, as that?ll OHKO Blissey after it has been hurt by Stealth Rock and then recover 1/16th of its HP due to Leftovers. It requires some number of EVs though, so it may not be worth it here. Personally, I?d just stick with the first set, as Forretress makes a great physical wall and you should take the best out of that. Still, something with defenses as high as Forretress? after a Light Screen are quite exceptional and its HP is pretty solid as well.
Forretress @ Leftovers
Bold ? 244 HP, 128 Def, 136 Speed
Sturdy
~ Toxic
~ Protect
~ Zap Cannon
~ Stealth Rock
A fun set mostly. You?d want to either badly poison something with Toxic and stall with Protect afterwards, or nail those pesky things that try to come in and set up on you. If you can predict a switch to one of them (Salamence, Gyarados, etc), you can try to nail them right away with Zap Cannon, after which you?ll outspeed them (provided Salamence uses a neutral Speed nature). Unless Gyarados runs max HP and Leftovers, it?s also going to get 2HKO?d by Zap Cannon. If you see the chance, set up with Stealth Rock and spend the rest of your time stalling with Protect. The fact that Forretress is free set-up bait for the likes of Gyarados and Salamence usually makes it a bad pokemon, but both Toxic and Zap Cannon can do something about it: Toxic means the opponent is eventually going down, limiting the amount of turns it can spend towards setting up. Given Forretress awesome Def, even a Dragon Danced Earthquake or Stone Edge isn?t going to do much. Zap Cannon paralyzes them and thus halves their Speed. That should buy you enough time to switch to something else and rape that pokemon afterwards. The downside is that Zap Cannon is so damn inaccurate and that some physical sweepers may carry Fire Blast with them (Garchomp, Salamence, etc). It?s a risky set, but fun to say at the least.
Forretress @ Leftovers
Impish ? 244 HP, 252 Def, 12 Speed
Sturdy
~ Spikes
~ Stealth Rock
~ Rapid Spin
~ Explosion
A Forretress based purely on setting up Spikes and Stealth Rock, or removing those when they are active on your field. When your job is done, blow yourself and something of your opponent up and start with a fresh and new pokemon, while your opponent has to send something in that?ll instantly take quite some damage. Beware of possible stat-boosters, as they can easily set up on you, survive Explosion and rape your team afterwards. Spending some EVs in Atk might not be such a bad idea. Make sure to take them out of Def and not HP.
Other options: Nothing that hasn't been mentioned in the sets really...