Lol, from reading this, I've seen someone who has said that: "Resistance" > Immunity.
Okay, firstly, you say that Ac can eat magic and become Immune to it, okay, so he eats [Widen Magic] CRY OF THE BANSHEE, TGOALID, he becomes Immune to Instant Death magic, he then dies.
Shalltear was immune to Instant Death Magic as well, but she died, she doesn't even have the concept of Life anymore, yet she still died.
And this isn't even taking into account Ainz's intellect, as he over-estimates his Enemies, look at the Clementine saga where he used several scrolls just to gather information and to cover his tracks, not even taking into account that Ainz will summon Multiple Minions to fight for him and can even then use the Cash Shop gear items to which into Perfect Warrior and fight him in Melee, and Ainz has shown to be able to fight well in Melee.
He fought Shalltear in Melee who's weapon is designed for it, not even taking into account the Hordes of Allies Ainz would have and then if things got too bad, he'd use the Staff of Ainz Ooal Gown summoning more and they would summon the Forces of Nazzaric using Gate to bring them forward and fully geared.
Now you keep saying: "But he's immune to the source, Magic." Fantastic, the problem is, So was Shalltear, she was immune to the Source Magic, Death Magic, but it still works, and before you say: "He'd eat the Magic" fantastic, it would still fail to stop his own Death. You see, no matter how tall you build a wall, it's useless if you can just go around it, that's what TGOALIS does, imagine Resistances in this case as a 2D wall, very tall, now imagine TGOALIS as a 3-D being, the 2-D wall gets bigger by eating the Source and becoming Immune, pointless, as it's still being walked around.