This is Story Related and CANNOT be missed!
You get two choices to get Asmodeus' Seal. Either beat him in a dance competition or get him and his ex wife Beth back together.
From 36:50 (On Part 2) in the video, you will see i choose to get him and his ex wife back (For the Match Maker, Match Maker achievement).
Whicever one you choose, they both take a while to complete anyway so not one is really quicker than the other.