Hi JimTheMuso,
Don't give up, hang in there. I'm not sure just where you at, at Chasm Ziggurat, however, according to your comment, I think you are just across the wooden bridge when the enimies armies are attcking you. If so try this: on the first try look for a defensible area to use on other tries. Back to the wall, back to a corrner, in an area with only one entrance. Stay away from the globs that explode - use them against the enemy. I've noticed a lot of shotgun shell laying around, thus, I think they think you should use a shotgun. For me I needed to keep the enemy a bay further away from me, so I used the machine-gun in short burst to the head. First burst takes off the helmet, second burst kills. Kill anyone on a higher vantage point than you first or they will take you out. Hopefully you have found a spot where the enemy must funnel in to get to you where you can take them out as they come. Once you thinned them out some get across the fallen piller and take care of the rest of the enimies there. That should take care of that. On to the next quest.
If you are talking about the battle againt the giant and Mathias, then, the giant is taken care of the same as the giant at Endurance, just make sure to keep killing the enemy that keep climbing up to help the giant. Mathias is a different story, see the post "I Cant Finish The Game" on how to take care of Mathias. One differentance for me on a PC, when the time was right I had to hold the "E" key down rather than tapping it.
Hope this helps. I have all the Tomb Raider games and have played them all. I find this the most challenging, rewarding, and fun. I hope this game is a hint of greater things to come for Tomb Raider, and look forward to the next installment.
Good luck