It’s not that hard to level in Champions Online at all, but there are a few basic tips to make your leveling easier.
Always pick a defensive passive. Regen, Lightning Reflexes, Defiance, or Invulnerability. Kill speed isn’t really increased that much using an offensive in normal leveling. In other aspects of the game, though the right offensive passive can help, a lot.
Group leveling is not worth it. It’s sad to say, but the exp bonus for grouping isn’t that much. In Champions Online, mob kills are a tiny part of exp: the bulk is through quest rewards, and a group doesn’t speed mission completion. It may even make it harder since the game likes to pack many more enemies in the same help-a-citizen mission.
Lairs are also not good for exp. These are the big 5 man instances, or the mission pack Serpent Lantern. People run them in teams on very hard or elite difficulties to farm drops. Leveling normally via quests and help-a-citizen missions is faster.
Pick an AOE attack you are comfortable with and use it, a lot. Most of the game is dispatching small groups of weakling enemies led by a tougher one. Also pick a strong single-target damage attack for those.
Be careful with your build choices. If you want to play a healer, you will need to “waste” powers and maybe advantages or leveling will start to grind. Also, be very wary of picking a lot of cool powers that dont work together well, or are more hassle than they are worth.
In leveling, Circle powers are useless. They ask you to stay in the same spot for some very nice bonuses, but leveling you are too mobile to make full use of them. Pets are also useless, until they go through with the pet pass. Currently they have too little HP and do poor damage.
Don’t be afraid to drop quests you havent finished by the time you’ve done the majority of an area. A common quest like this is “Kill 150 whatevers.” Those quests generally outlast all others from the same quest hub, so feel free to ditch them. You want to always strive for quests 2 or 3 levels higher, as they don’t take much longer than same or lower level quests, and give more exp.
Always upgrade your bags. You can buy them at the auction house in Millenium City if you don’t want to craft.
CO leveling isn’t hard, and you can break as many of these tips as you like. It just takes longer. That may be a plus, since I’m not even at a full 24 hours of playtime and am 14 levels from cap, but if you want to try a Time Trial of your own these help.