regice with focus punch and explosion is actually not that bad. focus punch ohkos tyranitar and does decent damage to blissey and enemy regices with a good atk investment. explosion speaks for itself. but i feel perhaps less people are switching ttar into regice now that superman is not the #1 team.
anyway as for EVs, mixed gengar is definitely more annoying to make a spread for and splitting EVs into his attack stat is actually really sucky. he needs to be fast and have shittons of s.atk evs to be worth anything as a special cleaner, and when he's mixed his durability absolutely plummets which i suppose isn't important for everyone or every team. as for regice, you can max HP and it doesn't really matter where you put the rest of its EVs honestly... i usually put most of my EVs in s.atk and a bit in speed to be faster than swampert, and sometimes i make my regices modest. it doesn't really need special defense EVs and i don't usually bother with defense EV's since i never find myself needing them anyway. regice is there to take on water types, electric types, gengar, and some celebis, and it's much more beneficial to give it s.atk in those cases so it can do more damage to them and to whatever is switching in on it, because honestly it is tanking hits from zapdos and gengar like a champ no matter what the spread is. but it doesn't NEED a lot of special attack and you can really pump its attack to your heart's content if you're using a mixed set, i think, but i would keep a quiet nature or something if possible. if you're giving it explosion over rest in addition to focus punch you have to worry about defenses even less imo.
also you can try giga drain on flygon but it's definitely something that's been tried before. give the competitive community some credit. i think if you try it you'll find that swampert is not actually as common of a flygon switch as you initially thought, nor is it a particularly good one. stab earthquake does a bit too much for comfort considering the fact that flygon is rarely a team's main threat to swampert, unless it's CBed in which case cb earthquake demolishes swampert anyway. things like salamence, gengar, skarmory, celebi, claydol, enemy flygons, etc are all much better at switching into flygon and shutting it down or scaring it away. without some combination of fire blast/hp bug/hp ice(underrated!)/sub/toxic/screech in addition to quakeslide, flygon will not be versatile enough to be an offensive threat. considering how weak giga drain is i'm not sure how much s.atk you would need to kill swampert or if the investment is worth it. honestly 4 attack flygon with eq/rock slide/fire blast/hp bug is probably the only set worth using in this meta unless you have magneton or something. there are too many calm mind celebis and you can't risk being spike fodder for skarm.
focus punch is the best move because prediction and wimpy special walls
<&spies> FUCK i just SNEEZED and made a MESS