First thing to do is to have a Fireball party. Pick a time where most of your kdmates can get online (esp you're best mages) and then everyone fb him. Those provs which cannot make the party should aid out their runes so that you get maximum amount of FB's all in the same hour. You said you are getting 1/3 success, that's is GREAT for a fb party. Cuz this strat works good even for 1/10 success rate.

After the party, continue to have your provs fb him whenever they login. If you manage 1 success per hour you are winning. Cuz a fb will kill more than his birth rate can make.

Once he's below 2 peas per acre, you could have your better mages try Greed and MS. (storms too but most likely he has Natures Blessing cast). But only do these spells if your attackers/other mages are maintaining the 1 FB per hour.

As far as attacking him, if you have undeads bounce him but no other attacker shud hit him until you're ready for a land grab. Pump up offense on your strongest attacker, then once it's close to breaking him (even conquest) you could have your smaller guys bounce him just enuf so your big attacker can grab land.

Hopefully just FBing him makes him decide to try another kingdom, so you wont even need to do the hits but still plan on hitting in case he's stubborn.