When you click on a tile that isn't a monster, a number is displayed. This is the sum of all the levels of the monsters in the tiles adjacent to the tile clicked. Using deduction and some dumb luck, you can determine what most tiles have beneath them. Much like in traditional minesweeper, you can flag a tile as having a monster. This is done by holding your mouse over the tile to be flagged, and pressing A. Doing this puts a green "5" over the tile - you can press A additional times to cycle thru the numbers 1 thru 5.
There are just enough monsters of each level to get you to the next level, so its possible to clear the board by only taking out monsters of your level or lower. Game ends when the board is completely revealed!
HP - Hit Points. Decreases when you click a tile with a monster at a higher level than you.
LV - Current level.
EX - Total accumulated XP so far.
NE - XP required to hit next level.
In the interest of helping, I'm going to re-write the rules in a slightly less confusing way:
The object of the game is to defeat all monsters. Monsters are hidden under tiles, and can be 1 of 5 different levels, shown at the bottom of the screen from left to right.
When you click on a tile that has a monster, if you are the same level or higher than the monster, you kill it and gain XP equal to it's level. If it's a higher level than you, you lose an amount of HP dependent on how much higher a level it was than you.
(cont.)
All classes have their pitfalls, but I could get the furthest (area 6) with the mage. Nearly any challenge allows you to kill all the monsters from the safety of a wall or ledge with the fireball spell.
The worst are the floating skulls, but you can kill those by leading them back and forth right/left in a corridor, and throwing fireballs at the ceiling.