Euphorbia hermentiana will tolerate some cold but how cold do you get?

I cant find much info on pruning unfortunately except I do know it produces latex that can cause some irritation to most skin types.

I had one of these as a house plant and never pruned it.

I agree with seaburngirl, regarding all euphorbia as dermatitis producing plants. That said, you can cut it down to the lowest shoot at any time, and I believe they may even sprout from the base if cut right down? About the cold - I wouldn't risk it below 10C, but Tugbrethril is probably the best member to ask anything about this plant.

In my experience, E. hermentiana can take temps down to about 35º F, but not much below that. Brief frosts--of less than 3 hours each night, no lower than 28º F--can be circumvented by covering the growing tips with styrofoam cups, since they are the most sensitive part of the plant. Taking the whole plant under a patio or porch roof will provide even more protection.

As for pruning: they will stump sprout fairly readily as long as there is still a trace of green in the stump. Cutting back to old, barky wood is a little dicier, but still often has good results. If possible, cut at a restriction where one season's growth ended, and the next began, or at the origin of the branch that you are removing. Everywhere that you cut will become a permanent scar, so it is a good idea to cut where the scar will be at least partly hidden behind other branches.
Sometimes, with an old or funky branching plant, it is better to take cuttings. Again, cut at a constriction, dip the cut end in almost hot water to stop sap flow. Let the cutting dry for a few days before potting up. Don't bury the cutting more than an inch deep, using a stake or two to hold up very long ones.

Good luck!
Tug