A two-stage approach involving alkali metals
The alkali metals are the ones in the first column of the Periodic Table of Elements, and include, among others, Cesium.
All alkali metals, the heavier ones especially, react violently when mixed with water.mixed with water.
All the nano-bots need to do is grab some alkali metals, and dump it all over the enemy. The second stage is either suicidal water-carrying bots, or just some guys with a hose.
