HP Grass or Water will do sh*t damage because Skar is Impish and has crap spc att.
Well actually in 3rd Gen Hidden power is actually physical. So there is no downside to HP Water or Grass.
Also maybe run more defense on your swampert considering that Blissey can take all the special attacks fired at it and can Aromatherapy. Ya recommend what the poster above me said. EQ over Shadow Ball for those pesky Fire and Electric types. Meteor Mash can do basically the same amount, bar the good accuracy.