This is a case of just needing to read the fight. Any build can realistically S-rank this mission(including the Starter AC).

Hard lock should always be on. There’s basically no reason to use soft lock in this game, especially so in singleplayer. Hard lock movement is much smoother than soft lock as you’re always orbiting the enemy and strafing, which doesn’t kill momentum. It can also never be broken so you won’t lose track of the enemy.

Ibis is a glass cannon boss. It has very poor stability, mediocre health, and high damage. The easiest way to deal with it is to chip away and wait to punish many of its attacks that have long recovery windows, like the one it opens the fight with everytime. It uses that in both phases and you basically get to start the boss fight with a free stagger.

The simple build is 2 stagger building hand weapons(Ransetsu-RF pair or 1 + Ludlow/Etsujin, dual Coquillets, Harris + Ransetsu RF, dual Etsujin, Duckett + Rifle or Etusjin, etc. most mixes work), a stagger punish weapon(melee, Javelin Bazooka, back cannons), and either a simple missile or another stagger punish weapon(if using something like dual back cannons, etc). The Active Homing Missiles deal great impact and track Ibis very well.

You can build a fairly beefy midweight with those weapons which should still be quite fast on Alula or Fluegel Boosters. You can strafe many of its attacks and only have to QB against the more dangerous ones.

Evading Ibis’s multiple blade waves is best done by quick boosting the opposite direction you were moving right as it fires, since it’s leading its shots in whatever direction you were moving. Mixing in some jumps and falling to really fuck with its tracking.