Ainz's immunities and hax generally give him the edge, and although he's not nearly as experienced as Grim due to the massive age gap, he was not only a great PVP fighter in the game, but he's also spent time learning how to fight physically in the New World. He's pretty strong physically as well, given he's a magic caster, and he has experience fighting against both other magic casters and melee fighters due to his fight with Shalltear. Ainz also just has counters to a lot of stuff Grim could try. I don't even know if the Time Hall would be a good way to kill Ainz, since he has a ring that grants him immunity to time manipulation—granted, that was mostly just time stop in the game, but it's not like other abilities haven't gotten stronger in the New World than they were in the game. However, that's not a guarantee by any means.
Obviously, Ainz's trump card here would be The Goal of All Life is Death.
The real question here is: would Grim's scythe count as a world item? Here's my view on it: Overlord has shown that things of significant enough power can be considered a world item effect by the items, despite not being one. Wild magic used by the Dragon Lords, for example. Not only do the effects of wild magic count as world item effects, but we know that Ainz would be protected against direct attacks by them as well, due to the Vampire Princess bonus novel. While it's not canon, it was still written by the same person and is backed up by canon. The Undead Dragon Lord used a wild magic breath attack that would have destroyed Ainz's soul, but his red orb world item protected him. In canon, the world item was able to bypass the world isolation barrier of the Platinum Dragon Lord. If that's the case, I have to think that Grim's scythe would count as a world item. Likely, that would mean neither could use their world items against each other. Given who Ainz is and how he fights, he would not stay got a fight until death. He'd retreat after gaining Intel of his opponent's abilities, and after learning enough, prepare countermeasures and come back to take him down, unless otherwise unable to retreat. I do not think it would be an easy fight but I do think Ainz would win more times than not.