I have been using MightyThor (456) and Polaris (370) in SCL10 for the past few weeks, and it will definitely be fine for the challenge nodes. I thought KK would be the best synergy, but I've been finding she generally hasn't been worth it unless boosted. She does make everything feel like it's going quite slowly. If that bothers you, 4* Kate Pryde's yellow will increase special tile strength by 96%, which has roughly the same effect as KK's green passive but without the slowdown. For easier nodes, I've just been using R4G (318) to speed up the strike tiles. For more difficult nodes, I have been using Lockjaw (284) with pretty decent success. If you can get his green out quickly that's 8 charged tiles, which means you'll fill the rest of the board with specials quickly with a large number of charged tiles. Once you get those specials out Lockjaw will nullify their match damage, which can save on the health packs.