Just finished getting S rank on all of the arenas. For the last two levels I actually had a pretty easy time with my new strategy.
Pretty much what I did the whole time was grab a weapon and use the jump/Y attack on everyone, even the little guys. If there were no weapons than borrow an arm from one of the enemies. It made for great defense as well as offense with the spread damage. Plus it kills everything but the large guys in one hit. I found it especially great against those blue ghouls as you are constantly in the air instead of on the ground where they rape you.
For the tentacles it was the standard method. Charge them and the slash them asap.
Against the bigger guys when I had the necro I would use the jump splattersmash against them for massive damage. To save time only use execution when you need health or necro. Otherwise finish them with another smash or a weapon/jump/Y attack. Against the really big guys that move around on all fours, use the weapon A / Y move on them (takes them down to around half health) and follow up with a jump splatterslam which should either kill them or put them in the red so you can finish them off with an execution and get back some health and necro.
On both wicker and the last arena I finished with about three minutes to spare for the s rank. So I suggest you try out this method if you are having trouble. I think you will be just as amazed as I was at how easy and simple it is once you get the hang of it.