You could just write a mutator that changes the Fleshpounds rageThreashold from 360 to maxHP + maxHp;
This way you would have to kill it twice before it would rage XD
EDIT:
The Fleshpound on solo has [750 / 1500 / 2025 / 2625] health (each difficulty respectively). If you are playing with 6 players he has [1687.5 / 3375 / 4556.25 / 5906.25] which gets truncated (the decimals disappear, coding wise these are all ints). On a 60 man server he has [11812.5 / 23625 / 31893.75 / 41343.75]!!! Regardless of players or of difficulty he rages whenever he takes more then 360 damage within 2 seconds. So as you get better, level your perks and increasse the difficulty you are actually making it hard to kill him and he becomes more of a threat.
The best way to stop his rage (or increasse it) would be to edit the RageDamageThreshold (the code says its configurable XD). Now RageDamageThreshold is used to calculate when the Fleshpound will begin to charge at the player.
if (!bDecapitated && TwoSecondDamageTotal > RageDamageThreshold && !bChargingPlayer)
StartCharging();
So if he still has his head, isnt changing already and has not taken more then 360 damage in two seconds he will begin to change. !!!!!IMPORTANT NOTE!!!!! If he is already red you can put as many bullets as you want into him without causing him to rage longer, so making him rage after a level 5 medic can quickly take him down.
The best thing to do is just increasse or decreasse the RageDamageThreshold to any number you want, 0 I guess would keep him in rage mode 24/7, 1 would put him into a rage on ANY damage, and 80,000 would keep him out of rage mode on a 60 man server!
Other Info:
Rage Time
Min : [4.25 / 5 / 6.25 / 7.5]
Max : [9.35 / 11 / 13.75 / 16.5]
This chart basicly means on Beginner he will rage anywhere from 4.25 seconds to 9.35 seconds. This is regardless of the amount of players. FRand() returns a random float between 0.0 and 1.0. Use 0 for Level.TimeSeconds as that is only used to tell the game what time to stop his rage (where for us, regardless of where it starts, we want the duration). The DifficultyModifier is [0.85 / 1.0 / 1.25 / 1.5] respectivly.
RageEndTime = (Level.TimeSeconds + 5 * DifficultyModifier) + (FRand() * 6 * DifficultyModifier);
Legal:
All this info is on your computer so I hope TW wont hit me with a "this is our code don't share it with anyone" kinda thing. Also if you are wondering how I got the numbers so fast, Excel document I made =).