In that case, you want to do a lot more than brick it; you want to destroy disks, ROM, and in many cases even PCB boards.

Explosives will destroy, but recognizable parts will survive. Your safest bet probably is burning your computer, fast and thoroughly.

I think white phosphorous is a popular choice for this kind of thing because it’s relatively easy and safe to transport, store, and deploy (compared to, say, strong acids such as aqua regia)