i dont think whammer is completely useless w/o a gem offensively, it just takes a little more effort to ohko 252 hp tar (i expect 252 hp 32 def ttar to die out b/c metagross's meteor mash was the main thing it was supposed to live, which is now 90 bp). 1), you need a brave nature and ~44 attack evs or a quiet nature that reaches the same attack stat if that makes you feel more comfortable 2), you need a turn or two of hail damage, or equivalent chip damage (the ladder is incredibly easy to accomplish, though you shouldn't depend on the former) and 3) TR's gotta be up (obvs). if you play smart, you can easily snipe or criple ttar. also, it ohko's politoed, which is obviously very nice.
defensively, i don't think you should worry about keeping megasnow alive like you would with basically anything else with ~90/~105/~105 defenses. the only reason those defenses matter is because of stray mach punches or weaker neutral moves, which is admittedly pretty neat, but you're not going to survive an attack anything that has a fire move or close combat without a sash, which is something megasnow obviously lacks. if you want to play it safe, use sash abomasnow with giga drain. if you want to go balls to the wall and stomp everything that is weak to blizzard or grass STAB (whammer), use mega abomasnow.
