Pretty decent tactics for someone not in the bata, you catch up really fast
Btw it's not that easy to place a nuke in the barracks, if they've mined the entrance you might find yourself a bit dead before reaching the pedestral 
Mining the tunnels is also a good tactic. As for the AGT, its underpowered in the demo. The real AGT does a LOT more damage. However a flame tank rush (3-4 flamers are enough) can take it out, provided you got the element of surprise on your side.
Oh, btw you can escape the obelisk's blast if you time your actions and jump/strafe at the moment it fires.