The numbers on explored spaces show the total level of monsters in the 8 spaces around it. Mark squares with numbers to remind yourself what level of monster is on that space (using deduction). Later you'll come back and defeat monsters of specific levels, because you want to get to exactly 0 HP before you level up.
Oh, I never noticed it was always in the corner; that really reduces the luck needed, as finding the 11 as soon as you can kill it makes the game a lot easier.