No you don't have to play Resistance. Resistance is the best Team to finish GW consistently but it's not the only way out. You can also build a Phoenix team you'll use to go through most of the nodes while you'll keep your killer teams for node 11 and 12. Before my Resistance team, I used to send two killer teams on nodes 1 and 2 so they build turn meter, then Phoenix squad on nodes 3 to 10 and, finally, my killer teams back on nodes 11 and 12 with a TM advantage. Not as consistent as the Resistance but it still worked pretty great for me. But that was before G12 and CLS...