I found that William is quite similar to Beowulf in DMC3. Powerful but slow, and without the projectiles. There are two ways to fight Beowulf. Get up close, dodge when he attacks, and then counter. Since William's attacks are generally slow, I don't think would be difficult to dodge with Dante's agility.

Beowulf also goes on all four in his second form, too, and is much more powerful (since he is much bigger). That doesn't stop Dante from attacking... this is basically what would happen when Slow meets Quick... If the slow one cannot touch the quick one, no matter how powerful he is, it will be no use.