In addition to the level-less progression system and improving equipment, your characters into Unto the Breach can grow stronger through additional means, namely the Skill Mastery and the Proficiency systems. Let's take a quick look at each!
Every time you use a skill (including basic ones like Attack and Defend), you gain Mastery XP for it, at a rate of 1 XP per usage. All skills start at level 0, and require 5 XP to reach level 1, with the amount of XP increasing by 5 every level, capping out at level 10.
Each level of Mastery attained will result in skills becoming more effective, as well as costing slightly less of whatever resources they require (SP and/or OD). Currently the rates are set for +10% damage/healing per level (So double the base damage at level 10), and -5% cost per level (a skill will cost half as much SP/OD when maxed out).
As you can see in the image above, Lucy has reached Mastery level 2 with Fireball, level 3 with Quake, and level 7 with Shards. The bar beneath each skill name is the XP bar for that skill, filling up as you use them. The costs have also been adjusted based on the mastery level.
The goal of this was to keep each skill fairly viable as you progress through the game, as well as give additional flexibility for choosing the ideal route to victory. Also it makes sense from a realism standpoint, I think, that if you successfully perform a specific technique a hundred times on an enemy, you're probably a lot better at it than when you started.
Finally, when you actually level up a skill, you'll get a nice little animation during battle to indicate as such, as you can see to the right. Any bonuses will be immediately applied on the next use of the skill.
Following on the idea of realistically getting better at something the more you do it, the other way to become even stronger in battle is to kill stuff! Well, I suppose that's a given for a RPG, but in this case, the Proficiency system comes into play when you kill specific types of monsters.
Essentially, every time you successfully kill a specific type of monster (a Zombie, a Montug, a Basilisk, etc.), the damage your characters will do to that type of monster is permanently increased by .25%, or 1/4 of a percentage point. The kills aren't tallied until the end of battle, regardless of outcome (whether you win or run away), and only count if a monster is actually dead at that point (so if you kill one and its ally revives it, then run away from the battle, it won't count. Or if you win the battle, it only counts as one kill). The bonus is shared across all characters (yay teamwork and sharing insights!).
Again, I think it stands to reason, if you kill a hundred Basilisks, you're probably pretty good at it by the end, and start to know all their weak spots. And while monsters will continue to get stronger as you do, this should help expedite battles against familiar foes. Currently there's no limit to the bonus, but that may change in testing (though if you want to actually fight 400 of each monster type to get +100% damage, I say more power to you. Maybe I'll even make that an achievement if I ever implement those!).
At the moment there's no means to view your per-monster kill counts (only the total kills at the main menu screen), but that information will definitely be included if/when a bestiary function gets added.
That'll wrap it up for today!