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.
You need to use a mek to get the work done, the first thing you need to do is let it agrrow on you so shoot it with a dart or something then use use the mek to shoot rockets ate his head like the titanosaur method
Did a test tame on a level 95 on official. Not even 40 rockets knocked it out. Shot it in the head, and by head i mean the very front of its face everytime, about 8 seconds between shots to align myself and aim. Didn't KO. The amount of rockets is wrong.