My recommendation for the fight is to ignore the pit fiend's resistances. Pit fiends are resistant to the attacks of pit fiends - it makes sense. It makes it very difficult to run a fight, though.
My second recommendation is to use the summoned allies only as fodder for Irresistable Command. That's probably not terribly powerful, but it's the best we can do at the moment.
So, taking those two recommendations into account, Pit Fiend A vs. Pit Fiend B.
I assumed AC counted as a defence, because otherwise Point of Terror is useless.
[sblock]Round 0
Both pit fiends begin adjacent to one another.
Round 1 (A 350, B 350)
A wins initiative with 36 to B's 28.
A takes 15 damage from Aura of Fire.
A spends an action point to summon 8 devil legionairres.
A uses Point of Terror on B, and succeeds (46).
A uses Irresistable Command on B, doing 16 damage.
A uses Pit Fiend Frenzy, missing with its mace (34) and hitting with its tail (46) but no followup (31). B takes 17 damage.
B has AC 39, Fortitude 37, Reflex 33, Will 35.
B takes 15 damage from Aura of Fire.
B spends an action point to use Pit Fiend Frenzy, missing with its mace (39) and its tail (32).
B uses Point of Terror on A, and fails (38).
B uses Pit Fiend Frenzy, missing with its mace (41) and its tail (39).
B will regret not using Infernal Summons, but I wanted it to be a bit different.
Round 2 (A 335, B 302)
A takes 15 damage from Aura of Fire.
A uses Irresistable Command on B, doing 12 damage. (6 devils left)
A uses Pit Fiend Frenzy, hitting with its mace (39) and hitting with its tail (42) but no followup (33). B takes 28 damage.
B's defences have returned to normal. However, he now has ongoing 5 fire damage.
B takes 5 ongoing fire damage, but then makes his save (14)
B takes 15 damage from Aura of Fire.
B uses Point of Terror on A, and fails.
B uses Pit Fiend Frenzy, missing with its mace (35) and missing with its tail (43).
Round 3 (A 305, B 254)
A takes 15 damage from Aura of Fire.
A uses Irresistible Command on B, doing 19 damage.
A uses Pit Fiend Frenzy, criting with its mace and missing with its tail (43). B takes 23 damage.
B takes 15 damage from Aura of Fire.
B takes 5 ongoing fire damage, and fails his save (9).
B uses Point of Terror on A, and succeeds (47).
B uses Pit Fiend Frenzy, but misses with both attacks.
Round 4 (A 290, B 198)
A has AC 39, Fortitude 37, Reflex 33, Will 35.
A takes 15 damage from Aura of Fire.
A uses Irresistible Command on B, doing 18 damage.
A hits with his mace (46) but misses with his tail (43). He does 16 damage.
B takes 5 ongoing fire damage, and fails his save (9).
B takes 15 damage from Aura of Fire.
B uses Point of Terror on A, and fails (31).
B hits with his mace (44) but misses with his tail. He does 15 damage.
Round 5 (A 260, B 144)
Bloodied, B surrenders. He still has ongoing fire damage.
A also has ongoing fire damage, and has 4 minions still around.[/sblock]
B rolled poorly, but also should have used Infernal Summons like A did. B wanted to hit with his tail in the first round, hopefully poisoning A. If he had, and if A had failed some of his saves, he could well have been victorious.
EDIT: In retrospect, A could have used his move action to use Point of Terror in those rounds he used Irresistable Command. That would have swung the fight even further in his favour. Not summoning minions is not a real option in a fair fight.