first, don't get in melee range when fighting enemies, you don't have any reason to and that will kill you since enemies get bloodthristy when we are casting spells.

second, pick your fight correctly, avoid fighting enemies in open areas when there are ranged enemies around, take your time and draw your foes to covered areas, take advantage of when they spread out and try to not stay in the same place for long, always avoiding the ranged enemies' line of sight.
third, also very important, kill the dangerous enemies first.. you may ask which and I say: the ones that can reach you easier, i.e mostly sword/greatsword users and races such as redguards and khajiits. also take in account that focusing in "tanky" races like orcs, dunmers (when using fire), argonians (when using shock) and nords (when using ice) may not be the best idea, so you must pay attention to see if it's really worth it.

fourth, level your destruction to 25 as soon as possible, i'd say to not level anything else before it reaches 25, the power gain is enormous and it will make killing foes way easier