Synergy in Challenge Tier

It is by design. In the colosseum all characters used for the battle have synergy because of an ability each boss has called “this is getting out of hand.” The challenge tier behaves like a regular battle, while the colosseum is different because it’s a special game mode. I’m a f2p player, though I did purchase one pack of currency and one pack of gear to level up era characters early on using crystals. My IG and Luke are 20, so if  you’re 20 or close you should be able to beat tier one.