How often I need to go back for trainee set? I mean after 10-15 lvls or so?Go to Khenarthi's Roost and farm treasure chests for "Trainee" gear. Once you have all 7 armor pieces and weapon(s), do all the main quests, fighters guild quests and mages guild quests. Hit up some dolmens on your way.
That should get you to 50 quickly.
After that, beat Molag Bal and do Cadwells Silver and Gold quests. When you're done with those, you should be close to CP160.
Go to Khenarthi's Roost and farm treasure chests for "Trainee" gear. Once you have all 7 armor pieces and weapon(s), do all the main quests, fighters guild quests and mages guild quests. Hit up some dolmens on your way.
That should get you to 50 quickly.