Let's Play Ys 8 Episode 66: Monster Hunter 3 Ultimate
2This episode is focused on killing the Oceanos. The big bad Kraken that dragged us into this whole mess in the first place. Though I have to provide a warning now.
KILLING THE OCEANOS IS A POINT OF NO RETURN
some quests will expire but it isn't too big of a deal but on the other hand make sure you handle any sidequests you may want to handle in Dana's questline. You aren't exactly screwed out of completing them but it is going to be very awkward to do so later on.
With Oceanos Slain we have more or less completed act 2 of the game... even though for some reason he isn't the big boss of chapter 5 (why exactly?)
Personally I think Oceanos is one of the best bosses in the game I'd put him in the top 5 along with Brachion. I should warn you we won't be seeing the last 3 until the endgame.