Monsters' levels are determined when you first encounter them by yours, to maintain a constant difficulty throughout the game no matter what your level. The XP you get from them decreases as you gain higher levels in relation to them. For example, when I fought with UC monsters when I was level 100, I would get maybe 5 XP for beating them, since I was only level 60 or so when I first got to the castle. The only exceptions to this are the Islands Closest to Heaven/Hell, where the monsters are always L100 -- making it a great place to grind.
in the training centre you can get about 300 exp per battle so 3-4 battles there would mean your 1% to your target already.
Given what I said above, I'm curious as to how you managed to keep a constant rate of 300 xp/battle as you leveled up and beyond the L7 monsters.