Currently in Xenonauts 2, there are 6 attributes: Time Units, Health, Accuracy, Strength, Reflexes, and bravery

Time Units are a measurement of the time your characters have per turn. Note that movement has a fixed TU usage but weapons use a percentage of your TUs. Knowing this, TUs do not increase the amount of attacks you make per turn. They only increase how far you move i.e versatility.

Health is pretty straight forward. It is the amount of health have i.e damage your units can take. More health = better

Accuracy is the base chance to hit an enemy before any modifications whether they be added or subtracted values or cover multipliers

Strength effects your soldier's carrying capacity, grenade throw distance, and recoil management. While throw distance and recoil management is nice, the main focus is carrying capacity as more capacity = more gear, more gear = more options.

Reflexes effects the chance for melee hits and for reaction fire. Generally, getting close to enemies is very dangerous so while you may try to stay away the main focus is reaction fire. Reaction fire occurs when an enemy does any sort of action during their turn within shooting distance of one of your troops; it is not guaranteed to happen but with enough guns chance is no issue. Do note that Reaction fire is also effected by remaining TUs at the end of turn. This is probably my personal favorite as there is nothing as satisfying as completely locking down your enemy making their every move impossible.

Bravery effects your characters' morale. The higher their bravery, the less likely to panic and be effected by enemy psionic attacks they are.

Leveling is based on "progress points" earned. These are gathered during various actions in tactical missions or passively (and slowly) though training at base. The further past your character's base attributes you go, the more progress point each "combat experience" attribute costs. They follow diminishing returns as below:

+1 to +5 = 1000
+6 to +10 = 1250
+11 to +15 = 1500
+15 to +20 = 1750
+21 to +25 = 2000
+26 to +30 = 3000
+31 and up = 4000

Accuracy gains 20 progress points for every TU spent on firing your weapon. Weapons take a percentage of your TUs to fire, but if you are going for volume per turn then the rifle is the way to go as aimed shots take about 48% of your TUs per shot, allowing you to do 2 per turn presuming you do not move.

Again, note that it is a percentage of TUs. A character with 50TUs will exert 48TUs firing twice getting 960 progress points but a character with 70TUs will exert 68ish TUs getting 1360 progress points

In short, bring lots of ammo.

Strength is perhaps the most straightforward stat to level. You get 10 progress points for every 1 TU spent moving when over 80% of your carrying capacity. Each movement is 3 TU so each tile move will give you 30 progress points. This means your character only needs to move 34 tiles to increase their strength attribute for +1 to +5 or 136 tiles for attribute +31 and up.

This is to say, load up your characters with as much gear as possible without going over and make them do laps

Reflexes is maybe the easiest attribute to level. You gain 20 progress points for every TU not spent at the end of every turn even if it was used in reaction fire. The consequence of this is a xenonaut with 50 TUs can get 1000 progress points if they do absolutely nothing while a xenonaut with 70 TUs can get 1400 progress points per turn doing absolutely nothing.

Get your gear, we're going camping

Leveling this stat can be very traumatising. You gain a whopping 50 progress points for each point of suppression your xenonaut receives. You only receive said suppression when being shot at or otherwise attacked. This is probably the hardest stat to level as every shot the enemy takes is a chance they have to kill your troops.

Effectively, your soldiers gotta pray to their respective gods

There is no specific amount amount of progress points for any specific action to level these attributes. Instead, they increase as your other 4 attributes increase. I'm currently unaware of how to min-max leveling these other than min-maxing the other attributes. This is probably by design as both TUs and HP are arguably the most important stats.

So basically: Shoot, camp, lug gear, and get shot at to level these two stats

Play the game

Here we are at the end of the briefing xenonauts. I hope death by power-point hasn't put you to sleep.

You may have noticed a certain trend when leveling stats. Looking at how each stat increases, you'll see a sort of rock-paper-scissors event form.

The more time you focus on shooting for accuracy, the less you have on hauling that gear for strength; the inverse is also true. The more you do either of those things, the less time you spend camping to improve your reflexes. The only stat that can be improved at all times is bravery and that is only because it requires getting shot at. I don't need to tell you the issues with that.

My recommendations? No matter what, your troops are going to need to move from point A to B regardless of how long it takes. Take your time, practice patience. Shoot when you can but always spend the time to cover your team.

Preserve yourself
Protect your team
Provide your enemies "services"