Combat Leveling - For the first 200 levels, arguably the most efficient form of leveling in the game. This is attacking other players and gaining experience from the fights.
1. Get a weapon and Armor
2. Train your combat stats some.
3. Search for players ahead of you or close to your level to attack using the User Search on the Basics drop-down menu.
4. Takes 1/3 of an energy bar to attack another player.
(Look at the combat record of players you find in search (if they have lots of losses they are probably pretty weak and look at how long they have been inactive, if for a long time then there weapon and armor are probably at zero and much easier to take down)
***Players further ahead give much more exp*****
Use common sense, if you can almost take someone 10 levels above you then train some more or get a better weapon or armor and try again later on that person. Watch the hospital as well, people being dropped for exp near your level probably means they are good targets for you to try.
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JailBreak Leveling - Probably the least efficient use of condoms to level with this method. At higher levels in the game there are few targets to use combat leveling so this method is used.
Just sit and break out other players from jail. You get one try for every 25% of an energy bar. If you succeed you gain 1% of a level.
Odds
Chance to Break someone - 25%
Chance to be thrown in yourself - 25%
Chance you fail but nothing happens - 50%
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Leveling off the Bots - Another decent way to level once your combat stats are strong enough and you will need to get some points into those stats because even the weakest bot will be a challenge early on.
Look at the Help with Bots tab for more info on them.