Look if you're going up against monolith you really need to focus on physical and radiation protection since those guys hit hard with both. The x7 suit is probably your best bet here with that sweet 4.3 physical and 4.0 radiation defense, and it only weighs 4kg which is really nice when you need to move quick. Plus, having 5 artifact slots means you can stack whatever protection you're missing, which is super handy in tight spots. Now the diamond exo is pretty solid too with similar protection stats, but at 9.5kg you'll definitely feel that weight when you're trying to get around. Both have that 2.1 psi resistance which helps when those psychic attacks start flying around.

Here's something interesting - if you're more into running and gunning, stick with the x7 and maybe upgrade that stamina recovery first. The diamond's better if you like holding positions, just make sure to upgrade those weight reduction parts early on. Oh and either way, grab some flash or sparkler artifacts if you can find them, they'll help you stay alive in those nasty anomaly fields while you're dealing with monolith. Just remember, with x7 you can dance around more and stack artifacts, but with diamond you're basically a walking tank that can take more punishment from those electrical anomalies.