Care to post your set? I can hazard a guess.
Swampert@Chesto Berry
Torrent
Careful 252 HP / 4 Df / 252 SpD
Curse
Rest
Earthquake
Waterfall/Avalanche
404 HP and 306 SpD does seem pretty good. What is your At and Df after one boost? Ground and Ice seem like better coverage that with Water. Plus when you're so slow Avalanche will almost be at 120 BP. Seems pretty good to me. Virizion and Serperior would obviously chase you away but you don't have to worry about Serperior in Wifi. If you had to have a 4x weakness, outside of Steel and Poison, Grass ain't that bad.
Not sure what my EV's are exactly, but it's pretty close to max max SpD, yeah. I go for Avalanche over Waterfall. You lose STAB but gain considerable coverage (unresisted outside of balloon and Shedinja?) - and I think Avalanche has negative priority this gen so you'll always go last (not that you'd be outspeeding much after a Curse). But the stuff that usually carry balloons can't do anything to Swampert with a couple of curses (Heatran with HP Grass might be troublesome, but most don't carry that anymore; Metagross can't touch Pert; Dory can't touch it after a couple of curses, etc).
Virizion, Gyarados, Nattorei, Celebi and Breloom are obviously the biggest threats to this set but all can be handled. Life orb Latios can handle them all though pretty nicely (except for Gyarados, but there's lots of stuff that can take that down).
Yeah, Grass is actually a pretty good type to be 4x weak to (relatively speaking, of course), although Nattorei running everywhere can ruin your fun, and Virizion uses YOU as set up bait, which isn't fun as that thing is awesome.
I'm not going to sit here and claim it's the best set in the world, or a world beater - but there's something pretty satisfying about taking your favourite Pokemon and building a team around it, and I've really enjoyed using this Swampert in this gen.