In regards to the Omastar, there are several builds to use; although a bulky build, I feel, isn't super good. Nevertheless, Here's a build for you
Omastar@Assault Vest
Modest Nature
Ability:Swift Swim
EV: 252 HP/192 SpA/ 64 SpD
IV:30/30/30/30/30/30
-Scald
-HP Flying
-Earth Power
-Ice Beam
This is a very tanky set that is best suited for a rain team(which would also help with one of Mangnaton's weaknesses), the IVs are fixed for HP flying(obviously) to help deal with fighting types(bar AV Conkeldurr), as well as dealing decent damage to Mega Venusaur(who would otherwise wall this set), Although not enough to stay in on.