Spiral swords I use a lot, however they cost 2 bars of DT to use, however if used correctly they make that make by the time you get rid of them and they jump up your damage output by a fair bit. Usually let them get a few hits on the enemies then when they are close to being destroyed, use to attack your opponent (I prefer using Lock-on+Back+Shoot myself, I think it's called Blistering Swords but I am not 100% off the top of my head and my xbox is off).
For the boss strategies against Cerberus and Beowulf I have recorded a no damage run against both of them, I could of done better, particularly against Beowulf but I kept getting hit by one pillar and screwing up my no damage run so I just played if safer on him than needed (you can do a lot more damage when he sends his 5-6 pillars in a row by hitting him with Round Trip + Judgement Cuts in between but I didn't do that on the run I uploaded).
Cerberus you can either use Spiral Swords + Aerial Rave on his legs to knock him down, then go for his heads, I prefer to hit the one which calls in the ice storms first, as they are really annoying (that is the head with the yellow eyes I think), or you can safe spot in the left corner where only his ice storm & ice breath (which is UBER telegraphed and easy to dodge) can hit you, when the ice storm comes in use a combination or jump + darkslayer evades to avoid the ice. I usually go for jump, normal trick, trick up, trick down, evades the ice about 90% of the time if done right.
Beowulf simply roll/trick up to dodge his 3-hit string, you can stinger his pillars at him, you can also destroy his pillars with you summoned swords and when he does his "SPARDA!!!" move, DT and sprint behind him, then hit him hard for even trying that move lolz.
In case you prefer a visual approach, here is my SS run with Cerberus at the end of Mission 3, and a video with me doing just the boss battle against Beowulf, it might help give you some ideas.
Hope that helps.