4. I've not seen that. I'm guessing that's near the boss instance?
Have you come across Avenger Demon Tremor's yet? They are ridiculous in how they move. And they quite nasty. X_X
Yeah, do you know that bit just before you come up to the boss entrance. You come up a set of stairs (it's always the same) on the right side, then up on top of the bridge with a small gap in the middle. Straight ahead is the boss entrance, left and right sides have towers. Before you cross over that gap look behind and too the left, there's normally a wagon cart there and a door that is blocked off. That door is the Underbridge when it's there. It's like the keep zones so lots of small rooms and corridors but it's very small, takes me about 3 minutes to clear it.
About that pack, yesterday I had: Avenger Jailer Horde and not sure of the last one, Demon Tremor. Needless to say I restarted the game

Thankfully that's the only time I've ever seen a champion Demon Tremor pack. I can only imagine how fast they would be as the last mob remaining with Avenger and Fast.
Couple of other tips I forgot:
1. Don't click on dead bodies on the floor, it's can bring flying mobs swooping in that you to deal with. Saves on time.
2. The most rare packs I've seen in the field is 3, if you get that many and have not explored the whole field just head for the bridge.
Can anyone tell me if I'm building my wizard right - mainly single player act 2 nightmare and stacking int/vit and attack speed. What are the other best attributes for a wiz? Crit hit, life after kill etc worth it?
No that's fine. Crit is nice, but Crit Chance and Crit Hit Damage walk hand in hand so you'd need both to make any use of it. Life Leech is okay, but it degrades the higher difficulty you enter. Such as in Normal mode it'll leech 100% of what the value it steals is, so if you have 3.0% stolen on a weapon it'll steal 3.0%. In Inferno it's 20% so that 3.0% steal becomes 0.6% steal. Life on Hit however is very much worth it, Life on Kill isn't.