The devil greases up your hand
Angels make you break your back
To Kill a King's official lyric explanation
It was just like an idea of expressing that... it's like the easy way is, I suppose, the quick way and easy way isn't necessarily the good way. But the angel - referring to your partner - is the person that you want to break your back for, the person that you want to put in the hard work and the graft for.
But there's definitely easier ways to do stuff. But I think - with relation to that song - it's kind of like it's tough to be doing those shifts sometimes but, hopefully, you've got something to do it for.