There are a few items and a Slann ability that can help reduce the effect of the enemy mage which would be useful, but beyond that it is a matter of kill him before he can cast too much. The book is a 100 point item so the Archmage will be packing exactly nothing else, you are looking at 3 t3 wounds which isn't very much. Even if you suicide something into his unit and target all attacks on the mage, it is worth it. On Teclis, he is only t2 so even softer than a normal archmage.
A normal archmage, because of taking an item which doesn't allow them to take anything else leaves them open to any type of attacks that can target him specifically. Salamanders, stegadon bolt throwers (though these two allow a LOS roll) and our nice chameleon special character. You can snipe thise guy out of a unit because he'll have no armour or ward save unless his unit is carrying one. You can also use a suicide character, cheep but with the right kind of weaponry conbination to take out the mage, just as Strewart said. Even a skink chief could do this with cloak of feathers, magic armour and additional hand weapon. You should survive the elves ASF and then kill off the mage in your first round of combat. Your skink chief might get run down from resulting combat resolution.
Remember that he still has to match the spell's casting value to get it off with irresistible force. Try becalming him and that should help. Otherwise, the attack is still a template, you're you're going to get look-out-sir rolls for you characters; and it has to come from the mage's base, so they're probably going to be at least fairly close to you when they cast the spell. It's something that's going to be happening often; and a tactic (magess throwing lots of die at one spell). Also, there's the chance he's not going to roll that spell, since he doesn't know the whole lore. Not particularly likely, but the chance is still there.
a unit of spear evles fights in 4 ranks, 5 as a horde. since a skink or saurus for that matter will be in base contact with 3 elves, 1 of wich is teh wizard, he'll get 12 attacks from a non horde or 15 from a horde before he strikes back. since JSoD doesn't work anymore active magic defence s quite hard to pull off against high elves. becalming cogitation is your best shot porbably and in the case of a purple sun, hope it missfires or it rolls porely and move out of the way, if i understood magical votexes correctly they can't change direction. also, give him bigger targets to worry about and then revive them with teh slaan, or go for lore of light and get I10 all arround
This has changed in 8th ed. Now you do not need to meet the casting value for it to go off irresistably.
indeed, that might be even nastyer than the purple sun or pit of shades. you still have a chance of survival with those but since TG units are usually 17 models strong (16+slann) a grey seer can just cut that unit down, without any possibility of revival or "Lo,S!" and get brand new rats
When he rolls doubles the spell is irresistable and the feedback scroll says it can be used instead of attempting a dispel, which you can't do if the spell gets off irresistably.
It specifially says in the high elf army book that you must still meet the casting value of the spell and roll a double for it to be undispellable.
An archmage will rarely miss the casting value anyway uness he is just throwing his dice around willy nilly.