I mean the plot point is so dumb... what drives me craziest is that the fix is so easy: Have KARN be the one to bind the Glistening Oil to Elesh Norn and then Elspeth finish her off. He made Mirrondon, had the oil in him for awhile, it's not a huge leap to imagine that he'd be able to do that with magic.
No offense to gamers but I swear video games feel like they have rotted writers' abilities. (or maybe it's laziness) Like "oh, my protagonists have leveled up enough now, they can defeat the bad guys." There's no tactics or logistics or plans or calculations to anything any more.