Have faith! The awesome thugs of TNN can handle those clumsy boulder throwers.
I ran a few quick tests of a Ri with a heros blade, silver hauberk, bracers and girdle of might with an E9N6 bless and scripted Bless-->Barkskin-->Mistform vs 36 of the armored boulder throwers buffed with strength of giants and an E4N4 bless and the Ri won every time.
Usually mistform held up but even if it breaks you still have a chance since your body prot of 30 is going up against 12 damage from a strength of giants boosted boulder. Your head prot is 24 so you can forge a helmet if you want to boost that higher.
If you didn't go for an E9 bless you can make up for it by going with black steel armor and casting stone/ironskin instead of barkskin.
Usually brands are great weapons for glamoured thugs but in this case I'd lean towards a heros blade since it will one shot a giant and you want to kill them as quickly as possible to reduce the number of hits you take. The silver hauberk wasn't because air shield will help against the boulders but because it has good prot and low encumbrance, which means you'll have less fatigue post-buff. But you could certainly go with other heavy armors.
Note that this was a simple test where I just had the two sides fight - the main thing I was testing here was to see if the thug could survive sustained boulder fire. Depending on the blockers you're facing another option could be a snake bladder stick with an eye shield to take advantage of Agarthans only having one eye.
I'll also mention the really scary thing is Agartha's H3s. If you don't yet have access to MR boosting gear/spells sooner or later (probably sooner) smite will pop your mistform and the next smite may well kill the thug.
Btw, kianduatha's point about friendly fire is a good one. The same thing will happen if the thug is surrounded by enemies. Swarm can also be an effective way to cause friendly fire casualties.
PS - Cold blooded creatures suffer absurdly large penalties when operating in cold so leverage that as soon as possible.