With 0.01 taming, level 9999, and with cooked fish meat, this takes 93 years to tame

The best way is to use the Tek skiff which you unlock after the boss fight(like the saddle) and have cannons mounted on it. Probably easier with multiple people

Use a bloodstalker and a rocket launcher, jump around to its face and shoot it with rockets until it knocks out. Jump off the stalker and land on the whale to quickly feed it.

To tame these, you have to learn Whãle from, Dory.
Ahem.
"Ooo Ooo" "OoWaoooo" "Huff. Huff."

"Incomprehensible Whãle Noises"

Then Wåla! It is tamed!

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.