and they do not need to be 900 ilvl to earn proper xp from WQ so it is a valuable way to level up a champion if you are in a hurry.
Else gray -> green -> blue is quite fast anyway.
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Maybe you should pick a champion with special abilities more than an actual "follower" since you cannot really control them.
As a mage, I think the best champ for this slot is Vargoth. 20% damage buff is quite valuable !! and it is others-friendly too. cause everybody around can get the buff.