There’s not really any reason to arbitrarily kill enemies on the map, you don’t get bonus XP for kills, just the soldiers that kill them will get a bigger portion of the objective XP than those that didn’t.
Dug into the code. Here’s the exhaustive list of what you get “contribution” points for. Which are then used to scale how much of the objective XP you get shared between your guys.
- Killing Something (Based on the deployment cost of the object, so bigger = better)
- Dealing Damage (Damage over time is counted)
- Healing (HP healed x a settings multiplier)
- Taking Damage (Health lost x a settings multiplier)
- Opening a Crate (flat settings value)
- Activating a Willpoint Objective (the white areas with the head icons, flat settings value)
- Mind Controlling Something (flat settings value) Mind control doesn’t appear to be hooked up, it’s probably using the ability modifier instead, maybe this is legacy code?
- Using Abilities (amount depends on the ability)