How to defeat Lord Reonall on crack Options

[Patrick_hindman]

Hey guys its been a long time since i've played because uni work got the better of me. I've started a new party multi-class fighter/mage, riskbreaker, beserker, necro, priest of lathander, and cernd(auramaster). The party i've got is pretty high level with both fighters and necro at level 13(verging on 14), and i've taken care of most of the quests in chapter 2-3 but i cant complete the fighters stronghold because i cant kill lord reonall and his army, lord reonall is practically invincible, his physical resistance to all forms of damage is extremelly high(i can only get at max 5 damage with any weapon type i'm weilding) and when he has hardiness hes physically immune, he's apparently immune to all spells 5th level and lower so breach doesn't work, has got alot of hitpoints and high regeneration. Most of his army can be disabled through enchantment spells but even they are tough with each of reonalls guards each of them able to use hardiness(though are vulnerable to magic at least), golems and a high level mage to boot.
Really this battle is crazier than the shade lord battle, need some advice or do i need a higher level party to beat him.

[Kerkes]

Question - is Roenall immune to all 5th and lower lvl spells? What is he, a Lich on steroids ? I fought this battle only twice , I noticed he is immune to "Breach", but didn't bother with using magic against him. But I've also noticed there are other powerful fighter-type characters immune to "Breach" but not immune to, say, "magic missile" etc.