As many have said, just make the core an remotely activated bomb and done.
I thought his skill would used in a more simple manner, much of an explosive device's elements are just middleware for detonation, directionality and so on, usually there's only 1 or 2 things that are relevant for the explosive power, with his skill he could circumvent all that as he did with the city's core and just throw the main element that'd be spiced by it's characteristics, like if there were magic crystals you throw ice element and get an explosion that freezes, fire is incendiary, wind is extra explosion or whatever, you get my drift, the best part of his skill is the simplicity of use and versatility.
Thanks a lot for the scanlation ^.^
