lol if you're not switching miltank out vs zapdos you've probably already lost. being in a speed tie is even worse than being slower. that means you have a chance of dying to a non-crit even when literally using milk drink EVERY TURN. it's like the speed ties in +6 de lax mirrors are. even if you rest loop, you risk dying should you go first at an inopportune time. it's probably worse than that.
granted, if you get lucky, i might siwtch out a zapdos if i've used up like 10 thunders trying to kill a miltank. there's no miltank winning in that scenario though.
blissey to c is absurd. blissey is a high b. easily. and i hate blissey.
edit: so i went back and looked at what you said. there's a lot of falsely drawn conclusions. vaporeon is the one thing blissey DOESN'T beat. even with light screen. (although light screen+growl will do the trick, but thats retarded). a crit surf + regular surf will kill a blissey. so fish for the first hit crit, but you don't second time surf unless you get it. you just growth and let her heal. alternatively, you can just pp waste with growth and rest.
all that needs to happen in zap vs miltank is... miltank going first, then miltank going second. that's it. if that happens, she's dead. forget the paras and the crits and the fps. you never try to stay in more than 1 turn with a fully healthy miltank vs zapdos, because a para thunder means she's useless vs snorlax. and that's the whole reason you run miltank. he also gets raped by ttar/nidoking at that point too.
blissey doesn't do shit, but most things don't do shit to blissey either. sure you can switch anything in against blissey, but nothing is forcing the blissey player to keep her in either. unlike miltank, blissey doesn't need to softboiled after absorbing a hit, where you can use that to gain a turn advantage. blissey can take like 5 thunders before being "forced" to heal.
think about all the stuff that actually has to switch out against blissey? flamethrower, toxic, ice beam, growl. you can screen too, but unless you have a marowak/charizard it's probably not too useful.
and how's kingdra your example of mixed attacker? nidoking, ttar, rhydon, dragonite. those are the pokemon you're looking for. and yes miltank is better against those. kingdra? what in the world...
mixed sweepers in general are two things: physical attackers that use fb/thunder. or special attackers that use explosion. under this criteria, i'd classify gengar and egg as mixed sweepers.
Conflict: most good pokemon are one dimensional. miltank? starmie? skarmory?
edit2: and when push comes to shove, i can totally picture a kingdra beating miltank with hydro pump + dragon breath, so that was actually a terrible example.