Few things to approach here. First off is that no legion games use remaining points of models to determine who wins (although it is used to break ties) so if you can play the objective better than him you will win, though that's a bit of a complicated solution that comes with experience. Secondly, how is he engaged with you all game? there would be at least 1-2 turns at the start when you can hammer him with the saber tank. its guns + beam + armour piercing shells gives you critical 1 and impact 5 on a bucket of good dice on 2nd turn and crit 1 impact 2 on turn 1. on average with an aim you should deal 7.5 damage without accounting for stealing tokens from allies, almost crippling it. Thirdly, get him to build the model. Most vehicles in this game have a hard time hiding for good reason so with it at full height it should be targetable from almost anywhere on the map (although it can also target you in return). Republic are able to get a good load of activations into their lists and with him investing in 2 high value models you will be able to out activate him, forcing him to use the AT-ST on your terms.