Try using Firestorm, in my magician character, that's my main attack spell. Coupled with a Crescent weapon +3 (or +5 if you want maximum MP regeneration) and Silver Coronet, you'll be almost in spamming range.
Plus Penetrator is really slow, even if you don't do much damage to him with Magic, by constantly keeping your distance you can keep doing pot shots with cheaper magic and eventually will kill him. Also, Insanity Catalyst helps to potentiate your spells.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
