Siegebraker runs were viable like that before the stealth Tyrael nerf. Now he is just a vamp healing annoyance. But if you still want him, get the kill siegebraker quest in act3, go to the yellow waypoint (rakis crossing afaik) and you will have him with you.
Tyreal is a BOSS! All you gotta do is go to Rakkis Crossing. Once you do, the Almighty Tyreal of Slaying is your companion for life (well, the rest of the game)! With his MIGHTY foam sword of destruction, he slaughters your foes with a very little (read: ALOT) of effort!