I think the key to balance would be to have each type of template specialize against certain monsters. As it is, mages/tamers pretty much RULE any PvM. Dexers can kill monsters, but not real strong ones, and not near as quick as a tamer or mage.

Just give certain monsters magic reflection and/or really high resist.

Also if you want to make dexers more useful in pvm, you could have monsters that turn pets against their owners. Like on Autumn Twilight, the Serpentine Dragon would command my wyrm to attack me. If tamers are less potent, dexers can make a difference.

Another suggestion is to make monsters perform paraylize attacks or cast paraylize spell, which would be frustrating for a running mage.