So, Quagsire... An acceptable bulk and a usable Attack stat. Terrible BST, but its typing gives it only one weakness, which is pretty good.
Damp: It's okay...but highly situational. Besides, with Explosion and Selfdestruct getting nerfed, this ability has found even less use.
Water Absorb: In singles, I guess it could be used to get a free switch in to a water type move...It's much better in doubles, where you can abuse it with Surf.
Unaware: Quagsire's best ability. Stops you from getting walled or sweeped. Really devastating against a Belly Drummer.
Powerful Attack + Free Confusion + Recover = Offensive Tank?
Item: Leftovers/Life Orb
EVs and Nature: Adamant (+Atk, -SpAtk); 252 Atk/108 Def/148 SpDef
Earthquake for incredibly powerful STAB. Waterfall for powerful STAB that will not hurt a non-flying/levitating partner. Leftovers combined with Recover makes you annoying to kill. Life Orb makes you easier to kill, but with only 85 base Attack, it wouldn't hurt to give an extra boost. Unaware lets you abuse Swagger, so you can put more EVs into that small base 65 Special Defense stat.
Quagsire, Destroyer of Grass types!...That aren't Ferrothorn...
- Mud Bomb/Sludge Wave/Sludge Bomb
- Acid Spray
Item: Leftovers/Life Orb
Ability: Water Absorb
EVs and Nature: Modest (+SpAtk, -Atk); 108 Def/252 SpAtk/148 SpDef
Like the previous set, but with a slightly different core move. Surf (higher base power, attacks everything else in doubles) or Scald (lower base power, but chance to burn), take your pick. Mud Bomb is there for STAB and the chance to lower accuracy, Sludge Wave is for type coverage against Grass, and Sludge Bomb is similar, except it has lower base power but won't hit your partner in doubles. Recover and the items are there for the same reasons. Acid Spray is the main attraction. 100 base accuracy, does some damage, and cuts foe's Special Defense in half. It's really the only reason Quagsire's Special set is even considered (well that and the whole anti-grass capability, but unfortunately, this also makes Unaware unusable.
Above-Average Toxic Set
EVs and Nature: Careful (+SpDef, -SpAtk); 4 HP/252 Def/252 SpDef
Like the title implies, its a better than average Toxic set. Protect is a must on any Toxic set. Recover lets you last even longer. Dig not only prevents you from having to switch from Taunt, but also helps stall for one turn. Leftovers has good synergy with Recover. Unaware also means no matter how many Swords Dances or Nasty Plots your foe has done, it won't make a difference.
Focus Blast could help you kill Ferrothorn. I didn't mention it because of its risky accuracy.
Sandstorm isn't too bad of an idea to slowly damage your enemies and increase Quagsire's Special Defense.
Amnesia is your best way to increase Special Defense.
Substitute is never a bad choice for a bulky Pokemon like Quagsire. Gives you time to boost your own stats.
Ancientpower has a small chance of boosting all your stats. If you do get it, go nuts.
Counter can really have devastating effects if you time it right. Just remember to give more HP EVs and less Defense EVs if you want to use it.
Curse will increase your Attack and Defense and lower your already hopeless speed. Not bad.
Encore gives you the chance to lock your opponent on a useless move. If you lock them on a stat boosting move, Unaware will make all those boosts pointless.
Aqua Tail is a strong physical Water type move. However, you can only get it in 4th gen, meaning you can't have Unaware with it.
Earth Power is an excellent special Ground type move. Too bad it's only in 4th gen.
Double and Triple Battle Options
If Quagsire has Water Absorb, anything with Surf. Also, something flying or levitating lets Quagsire use Earthquake.
Another bulky Water type who uses Surf and Water Absorb will create a chain of healing. Vaporeon and Lapras are good examples.
Ferrothorn, obviously. Just watch out for Focus Blast. But really, any Grass type that utilizes its Special Attack stat will massacre a Quagsire without Amnesia and out of Sandstorm. Bulky Waters can also handle Quagsire well. Although, anything bulky that isn't weak to either of Quagsire's STABs can take it on easily enough. Skarmory, who is immune to Ground type moves and Toxic is untouchable by Quagsire, does the job well.
I'm starting a Nuzlocke in Platinum where I will attempt to make all the nicknames I give Japanese words (Ex: Chimchar = Kaen [Blaze]). I will be starting sometime...this month...maybe next month? I dunno, I'm a really lazy guy sometimes. The comic I make based on it may never come to exist or be created very long after I finish the Nuzlocke, depending on my laziness.