Just started Sir Dan's story. God his supers are terrible/
His supers are definitely difficult, but his Level 1 is actually pretty good if you utilize this combo:
Square, Square, Up Square, Up Triangle, Super 1
This combo has served me well both in arcade and in online matches. I'm not a fan of his Level 2 Super, and I haven't even bothered to get his Level 3 yet. I've been able to get a good 2 or 3 kills with smart set ups and timing with his Level 1 Super. I wasn't too sure about him when I first started out with him, but now I actually like him.
EDIT: Also, Down Triangle, Up Triangle, Super 1 is another alternative, but a bit tricky to pull off because of the timing. Down Triangle is his downwards hammer slam, but it knocks the enemy up into the air at just the right height for your Up Triangle arrow to stun them, then follow through with the Super 1 for the kill.
That Down Triangle is also really useful for setting up people for Sir Dan's Level 2. I just killed all 3 enemies with it, because I launched them all up into the air at once with the Down Triangle attack. Lol.