After the cinematic, Lara hears Mathias's men approaching. Immediately run down the stairs and take cover. Either head for either set of barrels near the entrance or move around to the right side of the stairs, away from the ledge where the men are coming in.
The Solarii blow up the tomb entrance, making it impossible to escape the same way you came in.
If you use the cover near the entrance, you can take out a few enemies by shooting the fuel barrel on the ledge above on the right.
If you're positioned behind the blue barrels on the right, you can aim the rifle above their heads and fire a grenade, which will then land on the ledge.
Try to kill as many of the others as possible when they peer out. As you fight, more men will arrive on that ledge via the upper tunnel. A few others will slide down ropes from the ceiling.
You can shoot them off the ropes...
...or just wait until they land and blast them as they approach.