poison nailgun springs to mind, it stacks and enough hits it will hurt, well I don't know the current state of the game until I get it installed to see for myself. But when all else fails use poison modded weapons against monks, or piercing I beleive is rather weak on most monks, I could be wrong though. Also I do beleive that armor resist is locational, so learn the monk armor locations, for example the pants might be low in a certain resist, so hit them with that damage type in their legs, this will have the bonus of slowing them down, then finish them off by blasting them in the chest with a high damage weapon.
Neocron can be a bit more technical than just spray and pray, learn to shoot them in the feet