You use purification and fracture more, fracture requires 20 more TP for 30 more potency, after 7 fractures you've gained 210 potency and still have 160 TP over from the purification which is enough to keep youup untill the next invig, of course you lose out on some damage but it's better than dealing 0 damage due to TP. Unless you go for a build with around 600 or less SkS you're not going to be able to last the whole duration anyways. And with our current gear such a build does not exist.