At first you might think that this achievement is luck based. Fear not! I've come here to guide you through this achievement in few easy steps!
1) You must invent and equip charm ability for your dragon (thank you captain obvious)
2) Invent Imp Bunker Buster for the Juggernaut
3) Attack enemy province which have Juggernauts or wait until your enemy build one
4) Charm said Juggernaut
5) Start building Imp Bunker Buster on charmed Juggernaut
6) Tuck that Juggernaut somewhere safe and out of reach of your units (corner of the map is preferable)
7) Wait till Juggernaut become hostile again
8) Quickly charm it again
9) Fire Imp Bunker Buster
10) Achievement unlocked!