Usually with Harm, Mindblast or Wither as they are all cold spells and Fleshrenderers are weakest to cold. Don't Corpse Skin because it puts cold resist up by 10.
With 120 melee defense there shouldn't really be any issue going toe to toe with them, but if there is then you could do one of 2-3 things;
-(best) Use Command Undead to 'tame' 2 Skeletal Dragons from Ilshenar and have them tank while you cast, they're strong heal themselves, and can be healed. Alternatively don't forget you're in Doom and the Dark Father spawns Undead that you can 'tame' like Rotting Corpses.
-Use Summon Familiar - Dark Wolf - and a pair of bonded Bake Kitsune's (which are worth getting a good pair as they have high resists and 800+ mana) and do the same thing.
-If for whatever reason you can't do any of that, Summon 2 Earth Elementals and have them attack, don't waste time trying to Invis yourself if they are on you. Instead, learn positioning, Doom is the perfect place for this as the floor is tiled and easy to see where you/monsters/pets/summons are in relation to each other (unless you're playing the 'E'C). While any of your pets/summons are attacking and a monster is aggrod on you, move so that the monster has to walk on to the same tile as the attacking pet/summon and it will retarget them, providing you stay on the tile next to it, if you move away the monster will move off of the tile your pet/summon is on and resume attacking you.
None of this should be particularly hard and when you get used to it you'll find it pretty easy to just kill things while being melee'd, it works a lot better than failing to run and fizzling all your spells because you're trying to move instead of paying attention to timing. If, however, you're quite new to casting while being hit, which it sounds like you are, don't forget you can switch in and out of Protection quickly and easily. However, don't stay in it, because then you learn nothing and are 10x slower at killing anything, and can't combo. In Wraith Form you won't have the physical resist penalty in Protection because Wraith Form puts physical up 15 and Protection lowers it by 14-15 depending on if you're human or another race (because of Jack Of All Trades Inscription).