I’m playing two archer characters in parallel with a Velheider companion I bought since the very beggining. At level 65, both my characters are already higher level than my companion. I’ve stopped using any exp cards from quests, so my characters would not get the extra experience from them. Still, my companion seems will never catch up in level.
So far I have found companions only get exp from killed monsters as a percentage of the damage they inflict to them. So if your main character happens to be dealing most of the damage to the monsters, your companion will only get a negligible portion of the experience.
Spending silver in training your companion is useless. You would be requested to spend thousands of silvers if wanted to barely increase physical attack of your companion by 1. 
How am I meant to level my companion? Am I missing something, or are companions just intended to be useless in this game?
Also, I was thinking on going hunter C1 with one of my characters. Do you still recommend me going for it in knowing how useless companions are?

