Your defiantly not wrong, but this suppression does only last 1d4 round. If I was pulling this crap id enchant a few magic items with tongues and comprehend language, dispel magic targets the strongest and singular spells, and if the NPC didn't know that you needed it (Being a DM against this, my NPC's would I suppose, but id never allow it). That means getting rid of their language is a pain in the back side for sure.
I would assume the character would be wearing light armour (which they are still trained in as a cleric) so lets imagine this character has a dexterity of 10
I know it would vary at every point of progression, but if there level 20 some Magically enhanced (+5 deflection) light arour, so their AC is 19
Lets give them a +6 dex ring and a +5 natural armor. That is now AC 30. They would be the sort to stay back, so lets hope their team mate can (And still want to if they pull this




) cover for them. If they really want to find a way to get a +2 inherant bonus to dex also, we could call it AC31 (since the max dex bonus is +4 in chain mail) If I got something wrong, feel free to point it out ^_^.
So the magician would need multiple anti magic targets hitting him, the idea of an anti-magic field would be more painful to them though I should think. Using tongues and comprehend language is just a cheaper alternative to using a ring of evasion, which is a popular choice.