This is very easy to do in the later stages of the game. After you gain the ability to fly a helicopter and have infected vision, go steal a helicopter. Than turn on your infected vision by pressing right on the d-pad (
), and fly around. The infected water towers will glow and you can target them by pressing left trigger (
). From here just fire away at a distance! Now go find 24 more and the achievement will be yours.
Please note that I think in the later stages of the game, there is more opportunity (more infected water towers spawn). So I highly suggest just to wait until the later part of the story before grinding this out.