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
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.