I finished the New Game + mode on bastion and went to the Score Attack mode, but forgot to get the Turret Opera achievement. It can be done with the Gel Canister secret skill, but it seems the Gel is only awarded on the NG+ mode. Since I wanna make the Score Attack mode my final playthough, does the Gel Canister make an appearance on this mode or should I restart a NG+ to get it?
Turns out the Gel Canister is also available on the Score Attack mode. It's unlocked after the Arsenal is built on the Bastion. After getting twenty kills with one turret from the Gel Canister secret skill, the achievement was awarded.