Two 17 fuse together and became a Super 17! 2 health bars, making it a pretty standard for boss fights, just start beating on him until he gets around 3/4 to 1/2 health.
Once you get him low enough, GT Child Goku will show up to give you a hand, but in my case, it was more of a him showing up and for some reason, Super 17 had regain health, just to spite me! Start, once again, lowing his health to around the 1/2 point and Goku will go SSJ 4.
You have to defeat Super 17! After that is done, another wormhole will open up and suck Super 17 inside it. What are these wormholes, and why are they sucking up villains like Baby and Super 17?! You will have to find out during DLC Pack 2!