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I think the Dialga - Palkia is slightly more affordable than the Mew-Celebi one
The only real way to derail your combo is for an opponent to destroy Isochron Scepter or counter Nexus of Fate with an exiling card like Faerie's Trickery
(5/3/2019) If a permanent has in its mana cost, X is considered to be 0
Instead of running three card combos, maybe make sure your combos involve four cards, with none of them being your Commander
(Players dealt combat damage by that creature also get a poison counter.) Giving an attacking creature flying after it has become blocked will not cause it to become unblocked, even if the blocking creature can't normally block creatures with flying