In my Heartgold, My AMAZING Swampert knows Hammer Arm, Earthquake, Waterfall, and Muddy Water. I want to replace muddy water. Should I? if so, with what? I was thinking an ice move, like avalanche or ice beam. My Swampert has 210 attack and 161 Special Attack and is lvl 75. What move should I teach him?
Avalanche, as the difference in Sp. Attack and Attack is 49, so add that to avalanches 60 power to get a higher damaging attack than Ice Beam (if that's how it works.) Also, combined with hammer arm, you have a good combo. Use Hammer Arm to lower speed if your foe is slower than you to unleash Avalanches full potential.
You could go for Avalanche (much better than Ice Beam because it's physical) but then you have more chance of getting hit - well you want to get hit so Avalanche has power 130!!
Endeavor is a great choice for when your HP is getting low. Or, Swampert can also learn Outrage by move tutor.
You do have three great attacking moves already, so you could go for a stat move like Toxic or Stealth Rock.