Savanna Elves is very expensive deck and Miti Mumway is not very efficient. However it can serve a purpose against factions that give away a lot of magic or against decks that pressure to burn your deck very fast. Against the later, you can burn your deck very fast to keep up knowing that worse comes to worse you will be able to use your magic in a single card.
Miti Mumway’s damage is more scary than its life. Eleven life may seem like a lot but given Mumway’s very high cost it really is not that much. However, when fully boosted it attacks with six strength plus Trample, and it can be a really devastating way to take down gates and kill melee units. However because it needs time to buildup, the opponent has a lot of time to prepare.
One word of caution though, is to not be tempted to Trample past enemy lines and get Mumway into significant danger of being OTK'd. That’s a fast way to throw the game by losing your huge 9 magic investment. Instead, be patient, slowly advance him and keep him supported. On the other hand, once he has taken a lot of punishment maybe you can find an opportunity to just throw him in with Trample and do a final dive.