Attacking with Krallen
Krallen attacks in the following order:
So it's best to send in monsters with high health (like: Gorgo, Korath, D.A.V.E.s basically monsters to stall the defensive towers while Krallen Attacks). Krallen has really low attack so he will take some time to destroy a building.
Alway remember to bring monsters that target anything so they can target towers and help Krallen loot also.
Defending from Krallen
He isn't much of a threat, but an easy way to rid of him is an NECoRG/Death Traps. Always remember that Krallen is a looter. So that to your advantage, and make something that stalls them.
He is a back-up and support champion, so use him for looting. Also, to make the champion safe, let tanks take the damage and/or D.A.V.E.s which has a lot of health or even Grokus due to its massive amounts of health as support for Krallen to destroy resource buildings. Never let him deal with Monster Bunkers. Due to its low damage, he has to take a lot of time to destroy a high level building on its own. This changes in WMv3, because you can easily send Krallen and a bunch of Zafreeti to a lvl 5 OP defender to heal him up.