Oh, Bruce would most likely need a lot of time to prepare, but at the same time he is the best of them at disappearing and doing such things in secret. Besides who knows how many backup routines he has already set in the city. Best of all, he's probably the only one of them who most likely has several backup plans for stopping her in case Diana does kill her.
As for his role in the Trinity, I mostly agree. He acts with a plan and hits so low it painful to even read, as he can't approach the situation through the same brute force than Clark and Diana. My argument, though, is that doesn't make him any less dangerous than Clark or Diana, as in JLA alone, through that method he has taken out a bunch of White Martians, Eiling in the Shaggyman body, Prometheus and the Aztek warrior in the Obsidian League, just to name a few.