Basically you can get all three achievs on this Paramount War Scenario - Battle in Oris Plaza - utilizing Luffy and Iva

Unity Assist is the easiest, while unity clash is quite situational. So you will be aiming on getting the 10 unity chain while looking out for clashes.

Basically, anytime the opponent is performing attacks and combos on you, you can press the [switch character - LT/RT] button to interrupt their combo, and it will be an UNITY ASSIST

Unity clash procs when the opponent does an Unity Assist on you, and you press LT/RT. basically countering their unity assist with your own unity assist == unity clash.

Just keep attacking the opponent doing combos and eventually they will do an unity assist just like you would, then you have to press LT/RT right away when the opponents phrase pops (UNITY ASSIST) on their side!

This is what you will be aiming for while repeating the battle. You want to perform combos on the opponent, and every time you manage to land a combo, right at the end, press LT/LT to proc a UNITY CHAIN.

Its tricky, but this specific battle is great because your power bar is constantly at 3 so you can try these all the time. Keep doing combos and pressing LT/RT right at the end of said combos and youll get the chains. All while watching out for when the opponent does an unity assist, so you counter with your own for a unity clash.

Hope it helps. GG