It's all about the health regen per 5 items then the blocking attributes. Except the mage; she can use a slow down weapon or just pure attack power.
For those who don't really want to take the time to figure out the puzzle, just start from the bottom and work your way upwards as fast as possible (just spam the boosts). Getting the board reset usually heals up your champions a ton and usually lands you a nice batch of guards to add and treasure chests to grab. One time I reset the board a good 12 or more times in one battle. It was literally raining guards on my walls.
Here's my interpretation of the ending (and story): It's based off the Greek story except in this case he was "successful" in bringing her back to the surface. But at the end they realize you can't bring the dead back to life, no matter how much you think about it, mourn, or try. So she disappears. And because she can't come back to life, he leads a soulless existence (hence why his soul disappears too) forever spending his days mourning over the loss of his loved one (which mourning feels quite hellish) and wishing and trying for the revival of his loved one, but it is something that can't happen. The morals of the story are something like "If the one you love dies, you also 'die' because your soul is with her/him" and "You can't bring the dead back to life no matter how hard you try".
Just for people that don't understand the story, at least here's my take of it.
 "Insectonator"){kind=icon}
 "The Legend of the Golden Robot"){kind=icon}
 "King's Guard"){kind=icon}
 "Neil the Nail"){kind=icon}
 "Spikes Tend to Kill You"){kind=icon}
 "Blue Knight"){kind=icon}
 "Don't Look Back"){kind=icon}