I have one spider, and cannot start the beetle fight unless the spider is in either the first or the second slot from the left in the party. If it's in 3,4,5 (again, from the left) then it will not let me combat the beetle, but will let me start the Amalgam boss fight.
Viewing post in Idle RPG Spider Combat Simulator comments
Yeah that's currently the intended behavior. The beetle fight is a 2v1 fight so it only checks the first 2 slots (from the left) if they have a spider assigned.
The Amalgam is a 5v5 fight so it checks for ALL slots, even if it finds only 1 single guy assigned on the last slot it'll still start the fight.
A bit of a... spaghetti code, not even sure how hard it would be to make it so the fight just searches for the first non-empty slot in your party, so you could still start a fight regardless if you assigned your guys to slot 1/2 or to slots 4/5