Ok, data time!
I fought the beast five more times. Each attempt was done without taking the Ostmurk potion and while on Insane mode. I had, directly before the fight, 8773 XP.
Attempt 1: Skipped scene, no damage, used trap, ended with 10,819 XP.
Attempt 2: Watched scene, hit once, got trapped by Kayran once, hit with Quen once, used trap, ended with 10,950 XP.
Attempt 3: Skip scene, no damage, no trap, ended with 10,905 XP.
Attempt 4: Skip scene, no damage, used trap, ended with 10,821 XP.
Attempt 5: Skip scene, no damage, used trap, ended with 10,860 XP.
I am leaning toward the tentacles giving out random experience. However, the amount of XP Geralt has does not change during the fight / after hacking off a tentacle. It only applies directly after the scene when Sile lands beside him.
I am going to stick with attempt number five, for I believe it is a fair middle ground among all of the attempts (and it is the only one I saved afterwards). If other people wish to try their luck at this, please do so!