A trainer is a NPC that is used to level commanders. Trainers are mostly level 38 NPC's but are sometimes level 34 NPC's for lower level alliances who cannot afford to maintain a high level 38 NPC.
How to use a trainer[]
Trainers contain many units but like all NPC's, contain no commander so they can be crushed with ease. Because of this, you can use an army to level your commanders.