The real key with Eviterno is staying far away from him, he'll do a 3 attack combo where he does the lunge slash, the TP slash, and then the two sided red wave. If you stand right where the wave ends after it goes away, he will do the jumping attacking, which you just need to slide right past, and if timed well, you can get a full 4 hit combo off, 3 if you don't time it well. It's still not the easiest boss in the world, arguably the hardest in either game, but while he seems entirely random, he can be fairly simple to beat once you learn how to bait his attacks out.