Accuracy is modified by the summoner's current MP percentage (100% MP = 100% accuracy against a single target), and is sharply reduced by the number of targets in Zantetsuken (ability)'s area of affect.
MP percentage is what matters; not the amount of MP. For example, Zantetsuken (ability) is more accurate when used at 900/900 MP than if used with 1999/2000 MP.
Accuracy is not affected by the targets' or user's level.
When summoned against NMs, Odin deals darkness damage rather than attempting instant KO.
Damage to NMs caps at 9999 or 10-20% of their current HP, whichever is lower. However, percentage damage multipliers are applied after this calculation. This is why damage may exceed 9999.
Damage still depends on current MP percentage at time of summoning.