Mitosis is too slow and I doubt you can get out more than 3 of a certain type of creature in a regular game. This makes it much worse than shockwave, lightning, fire bolt, etc. The only use I really see for it is in fractal decks.
The only reason I made the cost 3|2 is that it has the potential to outdamage Fire Bolt and all other spells. A cost of 2|1 could work because it takes time for this card to be effective.