honestly, its doable with 5* event girls if you've pumped them up enough with decent Equipment and Ampies... but skill rate, atk increase, and critical (if not weaknesses) is your best friend
but still, why hold back when you can get your HP shredded faster than the non-boss Whaleship enemies (even the mid-boss that gives the 40% atk bonus for the ship is easier). my helenium, lycoris, and sakura are fully maxed on ampies (hp, def, and atk) and all 3 have 2 accessory slots with 2nd affection (4 slots for lycoris). They STILL die in 2-4 attacks....
Also, if we're facing lv160+ enemies, I feel like this is getting like star ocean where the enemies (and yourself) can go up to lv255. Then again, the game I'm creating has a lv cap of 300 (and Disgaea goes up to 9999 cause you can never have enough big numbers in that game)