Also undefeated in Ultimate (my loss was in Expert), but never in less than 8 rounds.

1) Full burst, down 35-40% depending on how well debuffs land
2-3) Slowly chipping down to stun
4-5) Stunned
6-8) Finish

2 Rounds stunned, 2 rounds overdrive, and there is no attack on the last turn, leaving up to 3 rounds to use petrify or poison. The first debuff is always countered by Amon, the first death is always resurrected by Andromeda. That leaves at most 2 rounds to deal with extra petrify. Poison can be countered with regen.