Hammer spell seem to need beating Cerberus in tower defense game.
However, I haven't got it as part of playing minigame when it happens in dream, but when starting it by clicking on cat by bed which appears after it.
At least that is when I got achievement for beating dog god and seen finally to have this spell.
No idea what is ultimate challenge achievement at moment.
And in case of worst, if having too much trouble or just tired of grind... In cheat engine search numbers as type double, not default option of 4 bytes.
Edit:
Hammer spell transforms hit enemy into another random one.
Ultimate challenge mode seem to be one of those plants I found and mostly ignored which make game harder for extra upgrade points. Forgot where I found it. In this case specifically, it is Boss rush mode. Opened portal to slime realm with all options on to see difference, when instead I got immediate boss fight, game kept going to next one until I finally lost.