Ainz is a very cautious combatant. Straight away he would use the big guns: grasp heart (which still stuns any opponent who resists it), timestop, and other high level magic. As Ainz's magic is of the highest order possible in his world (and even uses multiple verses), I'd wager it's enough to surpass Hassan's resistance. Ainz has over 700 spells. He can summon angels and undead almost as strong as him, use fire and holy magic, fly (does Hassan have any way to counter flight?), cut through space, produce black hole-like vortexes, and many more hax. Also, in a pinch, he will use TGOALID. It removes all immunities to death magic, and works like Hassan's evening bell in the way that it can kill those without a concept of death, but on a much higher scale (when he used it with an AoE attack, it even "killed" the air and earth around him, making it poisonous). It is clearly stated that it is impossible to resist it in any way, and that the only way to survive once effected by it is to resurrect yourself. They have very similar abilities, but Ainz has hax that break hax, so I think he wins with high difficulty.