To get the level 99 Sumner, go to the second level of the Battle Grounds, and play it normally. Find the exit, then backtrack until you find a bridge. It will be towards the front of the screen. You should not have to go far to find it. Enter the trap door and get all the coins. Note: You may need to unlock all the other secret characters first. When collecting all the coins, start on the right side. This level is very difficult and may require a few attempts. Sumner's stats are:

Level: 99
Strength: 999
Speed: 999
Armor: 999
Magic: 999
Gold: 5000
Health: 9999
Keys: 9
Potions: 9