If you BEAT THE HYDRO YAK at Battleground Level 120 before receiving the goal, and find that the game is not allowing you to complete Yak Attack (doesn't recognize all your completed battles), you may contact Support@TeamLava.com and get a ticket # in case you need to follow up.
But they said it was a KNOWN ISSUE and that they are working on a fix. It may come in the form of a game update.
My guess is that they will release a new version of the game sometime soon that will fix several different bugs and glitches all together.