The top answer to this question contains several references to "mid-battle leveling". I think this has to do with leveling up as a way of healing while fighting a difficult monster, but does someone want to clarify what this is?
Leveling up restores both your health and mana to full (and the former to a higher amount), which makes it among the most powerful of healing methods available. It also puts you in a better position against your foes.
Mid battle leveling is the practice of using this in the middle of fighting a powerful opponent. It consists of revealing and sparing several easily dispatched foes that contain just enough experience to give you a level up. It's primarily done when fighting a single boss, and in the case of special stages it is a way to preserve your potions for the second boss. However, it's not unheard of to use against normal enemies in the process of preparing for the boss.
Mid battle leveling often requires taking on higher level foes early on. The levels you gain from that means fewer of the lower level foes that you need to take on, so that you can leave them around for leveling up. You automatically have first strike on these foes, which means more health you can use to exchange blows with the boss. Depending on the setup, it may also be prudent to weaken some foes and use exploration to heal up.
How many monsters you use is based on your strategy. If you're really tricky, you can setup two or even three levels worth, though doing this while being at a level capable of taking on a boss is an occasionally difficult endeavour (but probably easiest in the Factory). For doing just a single level, often a single monster is sufficient.
Class choice can affect your ability to pull this off. Two of the more useful ones are the Assassin and the Paladin.
The Assassin has the special trait that lets it instantly kill lower level monsters. Since you automatically get first strike on them, it saves you all of the trouble of weakening monsters early, allowing you to devote a lot more of your health towards attacking the boss.
The Paladin also has a very unique synergy with mid battle leveling. It has a trait that restores a big chunk of your health whenever you kill an undead creature. By sparing undead creatures specifically, you can actually setup a mid-battle level stream that not only gets you the full heal, but also re-vitalizes you in the middle of the process. This lets you get even more hits in, and so Paladins tend to favour multi-monster components to their mid battle levels.
Mid-battle leveling (also known as mid-fight leveling, the level-up steamroll; related: experience catapulting) is the act of saving weaker enemies for a tough fight so that when you fight a much stronger enemy, you can kill them, level up, and reset your health and mana pools "mid fight" with aforementioned stronger enemy.
The tactic is part of several class strategies, and ideally you want to start attacking the boss around level 7 so that you level up (and reset your pools) 2-3 times before you finally kill it.