Well i first found out this some months ago, just didnt know how the hell i did it.
Now i've found out that if ya Cast a long taking spell, you can attack with melee while doing it.
While Casting and the mob is near, and ya want to do some DMG on the mob while doing it, Quickly press walk & run button once or twice, then quickly press Melee button. This should bug your character so that it "interrupts" the cast, but still doesnt.
Ya can see that your character goes to normal melee stance but the cast bar still goes on.
Hope this aint posted and that it helps ya ppl out there.
Note. This is usefull with if your cast takes more then 1 - 3 seconds, cant remember what would take thatlong but still
^_^