I think it was much harder for my Warrior then it was for my Inquisitor but hardly so difficult I'd have to wait to lvl 10. I think you're doing something wrong :/

If we could kill companions, Quinn would have been done in a long time ago

I used Quinn until I got Jaesa and fell in love with her but after many many quests and hours I found myself switching back to Quinn. I'm a simpleton and not having to micro myself is nice. Just attack without thinking while getting healed is nice. lol. but I don't think you can pick a "wrong" companion.