Right before the battle, you will be able to select the heroes which you want to put on your team. You will see that you have two options: Front and Back - in the Front should go the tankier heroes, the ones who can take a hit so that the back line with the damage dealers cannot die! In the Back line you should always put your most valuable damage dealers, debuffers and whatnot, because they are usually the ones who don't really have defensive skills and who are prone to dying easily in just a couple of hits.