Ok, so just shoot the balls of fiya at the enemy giants? I've really failed at using Magikill so far so any advice on the basic use of them would be helpful. I generally end up shooting 1-2 fireballs then ending up with a dead Magikill and it has lead me to the belief that they aren't worth the money unless the opponent just stands in one spot doing nothing :/
Against giants (which was your question) magikill are great because the situation is normally a line of giants hurling rocks at your forces - as long as you have a stationary target (say a giant or two) they dont move - put an electric wall where they stand and your hitting them all. If they stay still to keep up the fire on you, they are hurt by electric, if they move off the electric wall to avoid it, your giants are still throwing, his are slowly lumbering about so taking rocks to the head without returning fire. He has a choice of two very bad options. If he moves and attacks again, the second magikill puts a wall wher they now are - rinse and repeat.
If he moves from the wall then attacks with 1 giant infront (tanking) and the rest behind it, you electric wall the group and focus fire of other units on at the front one.
While thats going on, add poison spray from the second magikill, and keep refreshing it. Poison wont kill the giants very quick, but it hurts all his giants and a meric who's curing poison is a meric whos not healing a giant - good for you and bad for him.
Notes;
your magikill must remain behind a giant at all times - this makes him safe from the other guys giants - the giants simply cant hurl past the front line of giants, and archers simply cant fire past the giant 'meat shield'.
You need to watch for smaller units, like speartons etc walking forward of the giants to attack your mage - if this happens I suggest you manually target them with giants or what ever else you have got and dont forget to poison spray them.
You should have merics to heal, obviously.
Less obviously, you may find some spears a helpfull tool too - if a group of units comes infront of the giants front line, to kill your magikill, assume he makes it through the gap between giants, you pull your mage backwards (SLOWLY I know) but attack these wounded and poisoned units with your group of speartons as your mage retreats. He has poisoned and wounded units, you have healthy units because you are behind the giants and cant get hit. This makes it much easier to keep the mages alive.
The giants, spear, meric and mage set up is far stronger than giants alone, I think.
Put it from another side; I normally play chaos now, and if faced by a group of giants, merics, speartons and a mage - you can bet that the mage will be the first target for my medusa's petrify attack if I can get him - simply because he's the most threatening unit the enemy has.
Also, as a side, never underestimate the power of a mage for taking down the statue quickly. Put electric wall right over it, great damage to statue and any re-inforcements he spawns have to go through the wall to attack you. Hit it with blast while your waiting and its a great stylish finish :)
I dont know if two walls can stack on one statue, or if you should wait for the first to end before applying the second (assuming you have two mages).