Whenever I summon my minion, it always dies in a few turns because it is rank 1. I think there should be a series of quests in each world to level up your minion. It could receive more health, new spells, critical ratings, etc. This would definitely help in battles!
if you do a search on these forums, you'll see this has been asked before, quite a few times. This is also why they gave us new minions at level 75 that DONT die as quickly.