2 simple barbarian exploits I've been using for a while:
1. EDIT: This one was nerfed today.
2. No Escape (no cooldown and no disarming yourself) - Another level 30 feat ability in the berserker tree. But, this will NOT deal any damage. It's only to stun your target, and yes.. Without a cooldown on a 5 sec stun ability you can indeed hold other players stunned for an hour if you're an arsehole.
So, simply use one of your average melee swings, 1 2 3 q or e, and then hit your No Escape button right after you hit your attack button. There, that simple.
And now for the boss solo trick. (or just any other "elite" mobs)
Improving the health/stamina/mana regen in the skill tree is highly reccomended.
As you're attacking the boss, you will quickly notice that you're loosing this battle, your health goes down way faster than his own, or your stamina/mana is running out before the boss is going down. But no worries, AoC is not like WoW.. You can actually rest while a mob is after your blood. This is how you do it as a barbarian: attack the boss for a while, till your health drops down to let's say 60%. Now do Clobber, hit the | button (Toggle combat button) and rest! You can also use the No Escape trick I mentioned earlier over and over, if that's not enough, you also got Stunning Punch and Earth Shatter ( I think that's the name). As for you other classes, find your own CC abilities, and for the guardians; I think you can evade attacks easily. and you can rest while a mob is attacking you, as long as the attacks miss you.
If you don't have any CC abilities you can spam. You can also try to pull the boss on a not so huge cliff, jump down, hit the rest button while the boss is running down. (Mobs doesn't jump down cliffs, they walk around in a not so speedy way)