Instead of doing a Massive PvP which ignores the Overlord or keep on going (or do one fight then a second fight), why don't you combine the entire thing with a three way fight using a combination of the normal PvE and the Deadly PvP rules (so that this battle won't take forever)?
The caveat to this is that you a two combat areas per faction: one for fighting the Overlord (the Overlord Area) and one for fighting each other (the Faction Area). I'd use the example pages in the Rulebook until I (or someone else) can make an additional combat area for downloading.
Go back and forth (between factions), alternating players.
Place Tokens Step: Decide which combat area to place Blue/Red hit tokens and Green armor tokens. Place one token in EACH combat area for attrition hits.
1) Armor Step (PvP) using Faction Areas (Remove a Green Armor Token from your defense pool, remove an attrition and a red or blue hit token from other faction area).
2) Ranged Strike (PvE) using Overlord Combat Areas (Damage Areas are combined).
3) Ranged Strike (PvP) using Faction Combat Areas.
4) Melee Step (PvE) using Overlord Combat Areas (Apply Damage from Overlord to BOTH factions).
5) Resolution Step (PvP) using Faction Combat Areas, using Deadly PvP Rules (all red/attrition damage applies, don't compare the two values).
6) Resolution Step (PvE) using Overlord Combat Areas (Damage Areas are Combined).
If the Overlord dies first, it becomes a PvP battle between the two factions. If a faction dies first, it becomes a PvE battle between the Overlord and the remaining faction.
Winner is the survivor... If both sides lose, the winner can be declared by which faction did more damage to the overlord (or just call it a draw/loss).