This is a tough one to be sure. I styled my friend's Kenpachi wig, and I personally think it turned out pretty well....until he played "capture the flag" in it (don't ask). So unfortunately, there really aren't pictures of it, otherwise I'd offer some up so you can judge for yourself if you like this method.
But basically, we used a Punky XL from Amphigory.com and spiked it using a lot of Got2B Glued freezespray, and the styling glue on the tips to keep them from fraying. We sectioned it off into 11, using little rubber bands to keep them separate. Ths really can't be done without the assistance of at least one friend, by the way. Because you need to hold it up WHILE you spray it (a lot), gel the tips, and use the dryer (contrary to what a lot of people say, I have never had a problem using a dryer on wigs as long as the heat is on low). Of course, the bangs need to be pulled up into the spikes as well, since Kenpachi doesn't have spikes.
However, Kenpachi's hairline is....well, it's darn weird XD Unfortunately, I don't know much about altering hairlines yet, otherwise I'd love to help with that, too. And granted, I am very well aware that this may not be (probably isn't?) the best method for a Kenpachi wig, but I figured it never hurts to throw a piece of advice out there.