Kingdom Rush

Poison damage is inflicted by Ranger towers with the poison augmentation. These become available after level 5, so my recommendation is to go for this achievement on level 6 The Citadel - I built two archer towers in the centre of the map where all the enemies had to pass and as the level progressed I upgraded them both step by step until they were Ranger towers, and then I added the poison (top left option) to both and upgraded it as far as I could. 50 enemies is really not that many, the achievement popped well before the end of the level.

Alternatively, you could get this cumulatively as you play through the campaign - the poison ranger towers seemed pretty effective to me at whittling down the health of tougher enemies so you may well want to incorporate them into your general strategy.