I don't think the implementation of "risk/reward" and "meaningful choices" was very appealing to this encounter.
Players already start the fight in a "worse than normal" situation. You can't restore HP by any mean and behemoths are immune to all dmg types, except core dmg.
Then you can choose to play the game as it was intented, with damage types (part breaks, staggers, wounds... all those "little" things that make Dauntless fights enjoyable), but in doing so you get penalyzed by buffing the behemoths.
So basically the "risk" is to increase behemoth dmg and attack speed, while the "reward" is being able to play the game the way it was meant to be played...
MY SUGGESTION:
Let the fight start as a normal fight, no penalty at all. IF THE PLAYER DECIDES to make it harder he can break the pillar, he then loses the according dmg type or ability to heal, the behemoth gets buffed and whenever a pylon is destroyed the behemoth restores 25% of its own HP (to prevent exploits). So the player TOOK the risk and as a REWARD, for each pilar destroyed he will receive extra weapon xp and fragments of time (or whatever it is called) when the behemoth dies.
"Oh, but I destroyed the pillar and I can't kill the behemoth now..." Your fault, you took the risk!
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