1!
Collecting mana over the map, and dropping it ( click M ) to the builders

1.5!
Upgrading the towers (untill it's possible, then collecting mana as above)

2!
Some amulets can be usefull still from low lv:
FLASH REPAIR AMULET over all (defensive ability power, heals ally turrets)
Offensive amulet (like incinirate, lightning strike ... just a little more damage)

3!
Ranged attacks:
A little more is better than nothing... (more damage, and can slow the enemies)

4!
Try to figure out what your hero powers can do to help...
(For example, monks active auras ,not towers, can buff tower or hero allies...)

4.5!
Being a Jester and rolling bonus With Wheel of Fortuna.

5!
PETS!!
Having pets with aoe can be help your team too: Engineer (_); Imp (_); Cat (_);

going afk in ramparts isn't good xp. If you want to level, join cliffs, gather mana, give to builder, upgrade and repair, 0-60 in under 2 hours even if doing lower difficulty

Citic campaign is the best for exp bar none. That is the map you should be looking out for