It's rng then. if lucky you do 2-3 hits evenly dispersed on all the remaining enemies. if unlucky, all the 12 hits go to the lowest hp enemy...
But that's the chance of winning a lottery!
Plus, try this: put the playspeed to 1x before using tie attack, and see how much damage each hit does. Mine did 10k for non crit and 15k for crit a few days ago. It helped me to expect the outcome more accurately at least. In my fleet arena enemies usually have 50k~100k hp+prot, so tanky ships with almost full hp protection are very likely to survive
