A military comprised of highly-trained soldiers clad in 'outdated' steel armor whom favor Fabian-style tactics, with the soldiers utilizing powerful shoulder-mounted ballista, with said ballista firing bolts that possess tips laced with an unusual (and rather unstable) reactive composite explosive constructed using chemical compounds and conventional explosive material (most likely gunpowder), with the bolts having the capacity to detonate on impact with a target. 

In a 5th century RP.