If my opponent attack with his {Snapcaster Mage} and I convert my {Mishra's Factory} to a 2/2 creature and assign it as a blocker, can I tap it to turn itself to a 3/3 creature and kill the Mage?
It will kill {Snapcaster Mage} no matter what, 2/2 vs 2/1. If you're asking if you can boost it to save your land creature, yes after declaring as a blocker, you can tap for an effect and keep blocking. (Assuming it isn't labeled as sorcery speed only)
Edit: keyoto'd...
Thanks!