Theres an issue for me in Singleplayer where they teleport away never to be found again if their close to being knocked out using the Bloodstalker and Rocket Launcher method. The easiest way for me is to lure it near the land and fire a Cannon Shell from a mek. People say to use the Skiff and Cannon method but it might teleport away aswell.

More Astrocetus Taming & KO Tips