If another effect (or effects) modifies how much damage a source you control would dealby preventing some of it, for examplethe player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects (including Mjlnir's) apply
In my experience, the Game Changers that win the most games are the ones that generate persistent card advantage: Rhystic Study, Smothering Tithe, Necropotence
This legendary equipment costs three, equips for three, and has flash
For instance, if a card is limited to only 100 copies or less, it will be vastly more valuable than a card with an abundant print run
If you have an engine to keep swapping out your Pokemon and reliably bouncing them back to your hand, you can set up a pretty nice damage cycle with relatively few cards