I think it may just be that your list will not beat his, or most other heavily armored opponents, you dont have the ranks to stop a cav charge, nor the artillery to beat more than one unit of them. The salamander will do great against a unit (well not great but 1/2 wound and 1/2 are unsaved), but if he has more than one then you can expect it to run through your army.
I am surprised you are worried about the Great Cannon, you don't really have anything worth much that it can kill. Place your Terradons so that it can at most hit one of them. You should either deploy in cover or fly so that you can get into cover and out of LOS. If he does not have any other shooting you will have a better chance of getting to him. You have a twenty inch move. Even if you cannot charge him because no LOS, move next to him (well 1" away) that way he has to guess 0" and hopefully roll a 2 on the artillery dice.
Another use is just to run skirmishing skinks at it if its unprotected on the flank. It will take 3 turns, but he can kill at most 1 skink a turn and then you will win if you charge. If he decides to divert his forces to protect his machine then you attack the other part of the army. Plus depending on hit other shooting you might actually get there. And its only 70 points if you dont.
It could also be due to having limited terrain on your board (there should be a forest somewhere or a hill or a building to get out of LOS)
Really though against that sort of list you are going to have to play point denial, and try to win through table quarters as killing 3 knights to get half points on any unit is going to be difficult.
The way to handle the knights is to use burning blade of chotek

he cuts through them like butter. Put a skarvet on cold one with Chotek and a ward save and bsb, if knights get to close he can charge out of a unit of saurus and take them on himself, as well as ruin the enemies plans. Otherwise large units of Saurus to win through static combat res, and maybe a kill or two. Or large units of skinks.
To be honest though looking at your list how do you play with it normally there just doesn't seem to be anything that can actually kill the opponent besides 1 salamander, unless the opponent runs big monsters, which you then have a field day?