Spoilers - Whale Strategy
I figured out an easy way to kill that giant whale.
equip the shotgun you get from killing jack the ripper (might have to trade his soul for it)
Buy 60+ rabbit watches.
equip some of those revive draco rings you get from the item seller towards the end of the dreamworld whatever labyrinth bonus thing.
equip the andor sword for a bonus turn and the black rabbit ring for another bonus turn (might not need these)
I will say I was level 999 and had like a 5000 atk stat
What you do is in your first turn is use the shotgun, then for your three normal turns do omnibless or King's Order 3 times to buff your character.
If he doesn't kill you, do a shotgun blast, then go to items, do a rabbit watch, then go back and do another shotgun blast. Just repeat that over and over. You'll never end your turn to allow him to one hit kill you. Use things like the revive rings or gritt to survive long enough to buff your character, then just do shutgun/watch back and forth.
I did it with about 30 or 40 watches this way. You might not even need the revive items or the multi turn items if you are willing to spend time required to use up your max allowed 99 watches.
Now that I think about it, you could probably use this to break every boss in the game because it never lets the boss start their turn.