It is said that no one can match a fully matured Hellhorned Demon in combat. I have yet to see this proven false.Though I can't say that I'm seeking out that opportunity...
Due to the slay trigger affecting all friendly units on the floor, it is very good in combination with Floor Overstacking by ascending or descending other friendly units.
Furystone is effective on units who already have a way of gaining Rage because of the way factorials mathematically progress/regress (pseudo-exponentially, with Rage decay/regressing at a rate of 1 per wave).
Strengthstone is effective on units with Slay affixes because, in order to benefit from as many Slay triggers as possible, you must make sure that the unit scores the Slay, which is further guaranteed by having as high an Attack value as possible.
Frenzystone is effective on units with Slay for the same reason Strengthstone is effective. Multistrike has the added bonus of the possibility of scoring two Slay triggers in one wave, because each strike could hypothetically score a Slay.
Speedstone is good assuming Branded Warrior already has the damage to constantly get Slay triggers. Since Branded Warrior will most likely not be the front unit, your other units will attack before he gets the chance to. If Branded Warrior attacks first and manages to get a Slay trigger, the Rage will be applied to all your Units on the floor and they will benefit from the Rage this turn, rather than next.