Heres some old schoole invoker tactics I used with Infernalicus(NEXUS Leader):
Have them walk into your Firestorm then hellstream once and flee.
Rinse and repeat 2 or 3 times and watch the his EQ to explode, this will get your opponent angry.
If you keep fleeing he is gonna think he is winning when in all reality your blasting his EQ, getting him
mad and counting ticks, when he is about between 50% / 75% hp look to nail a dispel magic and a
final hellstream.
In the case of ferals and 'fury', flee every time it lands and wait off the ticks. If you stay in battle
you are only playing into their game, keep in mind a 'good' PK battle can last for hours of
back and forth tactics so there is no shame in the hit and run, we used to have some classics back in the day 
Hope the strategy helps, master it and prevail
