I like skarm because it walls most Mew better, as well as both SD lax and drumlax. It also takes Explosion pretty well. I concede that I am probably underrating Lugia though. In my experience Celebi has been a lot worse than Miltank, though most that I see run Psychic instead of Perish Song. Miltank + Skarm is capable of walling basically anything that doesn't run powerful special moves and paralysis is very handy for letting Mew / drumlax break through things or waste heal bell if you are stuck. Trading Miltank for Mewtwo or Snorlax is not such a bad trade imo. It also allows you to cover random Curse Ho-oh / Lugia that might otherwise be able to sweep you under the right circumstances, whereas I see Celebi as a more temporary measure (maybe because I have only faced Psychic versions) and it is also weak to something on nearly every setup Pokemon's set. Another issue with Celebi is that it doesn't really force Pokemon out immediately. If they stay in while Leeched and/or PSonged then switch on the last or second last turn, they might be dealing a fair amount of damage that you don't want to have to deal with. I know some Celebi have tried to force my Mew out in the past and they usually get the short end of the stick. Thinking about it more, I can see some potential in just using Steelix to attempt to trade itself for something more valuable. You talk about Zapdos paralysing things, which Miltank does also, which is one of the main reasons I have found it to be effective. Another thing I am considering: I actually lost a game to a Curse Tyranitar because I missed a couple of crucial Submissions and I think Miltank got exploded upon earlier. Is there a better way to handle Tyranitar with the standard ubers team than try to kill it with submission? It does seem kind of threatening in theory.