The trick is to work backwards from your winning move to calculate a strategy. Ideally, you want Discord to end his last turn on either floor 4, 3 or 1, so you can drain that many floors and win.
Thinking further, 2 is quickly identified as a 'dead floor': you've lost if Discord drains it to that level, since draining 1 floor is your only legal move, and he can drain the remaining floor to win. Floor 7 is the next dead floor, since no move will take Discord down to 2, the next dead level below. The 'dead floors', in other words, floors where you've lost if Discord manages to lower the level down to them, for this tower are floors 2, 7, 9, 14, 16, 21 and 23.
Noticing a pattern? Every dead floor is either a multiple of 7, or 2 greater than a multiple of 7. The rest of the floors are A-OK: as long as you can land on any of those floors, you can win.
All Luna needs to do is drain levels strategically to ensure Discord doesn't send you to a dead floor: if it happens even once, that's it. GG. All he needs to do is keep sending you to dead floors to win. You avoid this by making sure Discord is put on nothing BUT dead floors himself.
Since 23 and 21 are dead floors, Luna HAS to open the game by draining 4 floors of custard to put Discord on the dead floor 21. But from there, it doesn't matter what Discord does, as long as you drain custard to send him to a losing level.
Whew!