Astrocetus taming is currently a little buggy on xbox official version. They appear to lose 5-10% effectiveness for each piece of kibble they consume. I'm sure this next part is intended, but for every non-torpor inflicting cannon shot the whale loses a chunk of taming effectiveness. My reference is while taming a 100 astrocetus, it came down with 75% effectiveness, and as it ate kibble it eventually dropped to less than 20% effectivenes.
Singleplayer taming is so hard. I had nobody to aim the cannon, and I'm currently waiting for the astercetus to wake up. I struggled so hard. He's a blue level 8. LEVEL 8. Dododex says there's a 93% chance of death for the cannon for a level 8.. Pretty lucky, eh? Singleplayer taming is.. Hard. Not impossible, though. I managed!
Don't use mek because the astrocetus takes first the torpor and then the damage. The effectiveness will go down.
We tamed a 130 with mek and Kibble and get a 136 -.-
I think the stronger the weapon the more you loose the effectiveness, so the "best" way is to knock out with rocket launcher.
It's the same effect with rock elementals.
Wildcard should repair that.