you need helicaths a3 fully booked.
ideally, but not necessary, you want his a2 to be on a 4 turn cooldown unstead of fully booked to a 3 turn cd so that it stays in sync with his a3
The reason for the delay is that the boss is slower on NM than on UNM. if you open with his a3 it will wear off before the boss does his 2nd aoe
From DWJ's site; how the CB targets. he goes through champs in the order your selected them starting form the leader position, and the first one that gets a yes answer is the one he attacks:
- Does the champion have low HP/can he one-shot your champion?
- Are they negative affinity?
- Does the champion have these buffs on them – Increase Defence, Block Damage, Counterattack, or the Steadfast mastery (the Demon lord considers this as a buff, and it avoids it)
- Do they have the lowest percentage max HP remaining of all your team?
- Are they in the leader position?
Edit: to clarify he asks the first question of each champ in order, and if they're all "no"s then he start over with the 2nd question, goes through, repeats until he gets a yes. if he makes it to question 5 he just attacks whoever is in the lead