On Demon Battle screen, topright is
Day x
(Burst) Time Remaining
--:--
buff icons
Apply Effect

The Apply Effect button is where you can spend Grails. Please note that you'll lose them all at next reset and that only Union leader, Sub-leaders and Officers can spend the Grails. Grails are Union-wide after all, and all Grails earned by all members of the Union are given to the Union.