With a small force pitted against a large one, it would make little sense for the Templars to try to take the French army on in the open field (not that many small forces haven't put themselves in similar positions). The Templars' advantages are (i) they are rich; (ii) they are organized, well trained and well-armed; and (iii) acording to the rules of this timeline, surprise. They have to use these.
To defeat Philip's plan, the best hope for the Templars is to strike hard and early, i.e., sieze the king, his sons and as many of his counsellors as they can. Having done so, they can and expose Philip's scheme to destroy them and show the world how he intended to fabricate the evidence. After that, the usual pattern would be to blame 'evil counsellors' for the scheme and execute them rather than king Philip himself.
Alternatively, the Templars can try to get out of the country with as much of their wealth as they can, but that wasn't the challenge.