I personally like using
Code:
$npc->NPCSpecialAttacks(parse, permtag);
fABDINCMU will give it immune to fleeing, immune to Melee,magic,fear,root,stuns,charms,mezzes and slows. Just remember if you still want it to summon, Enrage, Rampage, Flurry, Triple and Quad, you still have to add those in too even when you make it invulnerable. To remove it, just do the line again after it receives 5 signals from the trash mobs on their death and carry on (do the same line, removing the fAB and whatever other Immunes you want).
Hope this helped.