As of the latest snapshot, very simple. You will need the following objectives:
mdeath : deathCount critiria
mtotkills : dummy
Use a rapid redstone clock for the following:
A circuit that locates non-player entities with 1 death using the mdeath obj. Then, with the operation command it adds this death to the mtotkills. At the same time a command block tests for a non-player entity with mdeath 1. If existing, it gives exp to the mob master. Right after all these have happened, mdeath for that entity is reset.
On the same clock a circuit checks if enough mobs have been killed to unlock new mobs using the mtotkills.
Finally, when the mob master tries to summon a mob, a short circuit checks if he has enough exp. If he does the mob spawns and an amount of his exp is consumed.
Hope this helped C:
