Coliseum

The game mechanic is explained in the description of "Now There Are Five Of Them" under Event Info.

"At the start of battle, up to 4 enemies (from the bosses perspective, so your summoned allies) are summoned based on this unit's era level (The boss's era level) and the lowest enemy's (your characters) Era level..." 

You need to level up your character's evenly. It definitely could have been more clearly communicated (as with many things in the Era's update) but it's always been written there in black and white. Both my Luke and IG are lvl 22 atm, and I got above 70% against tier 2 AT-ST today.