I reckon the best strategy against Nabighah is simply dispel his prot vs evil/stoneskin/desensitize/resists. This leaves him with ABS which really isn't enough. Then have a decked out sword spec with dual HumanSunders beat him up. He's going to take EVIS/DISMEMBER every hit of the swords (a decked sword spec exploiting a vuln easily hits OBLITS on unprotected). Throw in an 8-hit flurry and he has to flee in maybe 3 or 4 rounds.
A pair of high damroll mace specs would also be good. Preferably RBW. Also, high svs goes a long way against devils. Rupen could shrug off almost anything the devil threw at him whilst taking very low damage.
Sheer, overwhelming damage often takes conjurers out quite quickly - see Charine vs Rupen log.
For those pallies, it might be best to kill the elemental and just leave.
Or just have Clahier around... due to dismissal, Nabighah and Clahier are going to be evenly matched. Add in one gang partner (high dam warrior lagger being ideal) to Clahier's side and Nabighah will have to run.