[Crenshinibon]

It can disrupt casters, yes, but so can the Assassin with his poison, which, unlike the Wizard Slayer's ability, is useful again non caster characters as well.

In my opinion, a full assassin and a full archer are the best at stopping mages (and other characters) due to the poison (Assassin) and the sheer amount of attacks per round, along with damaging abilities (archer). Don't forget that they're effective from the start, unlike the other choices.

With the Assassin (or any backstabber in general) I find mages a nuisance as most can be killed within a single backstab, whereas the Wizard Slayer/Thief has to attack the enemy six times (and hit) in order to have them fail at casting their spell most of the time, and at that point, they should be dead anyway.

The only time that a Wizard Slayer could have an advantage of the Archer and Assassin is when initiating a fight with a full amount of attacks per round. Even then, nothing is guaranteed since there is an almost instant cast trigger on a lot of mages and boss characters, at which point there's still a chance that they might succeed in casting all of their protections.