In mid-battle leveling, does that mean you have to have revealed the whole map already?

I kinda get the point that what you simply do for say a level 10 boss is to:

1. attack boss until his next hit will kill you.
2. kill an enemy that will level you up(so you probably need to have 29/30 exp then kill a level 1 enemy)
3. attack boss again etc

I try this also but if you suddenly enter a space that you haven't explored yet, you do get the heal but the boss also heals(usually more than what i damaged him...so im back to square one)

Am i doing it right? should i had explored the whole map first and setup my mid-level battle? But that means i can't heal via map anymore and must rely on 2-4 health and mana potions. right?

No, there is no requirement to have revealed the whole map, nor even a significant portion. You might consider a start-of-game mid-battle level to take on, say, a level 3 foe by killing a level 1 foes and just preparing for the next hit. You should be able to do this by just finding few enough enemies around the start.

The only map revelation you need is enough to set it off and stand advantageous to defeat the opponent. This may simply mean exploring as much as needed to reveal your full chain of kills. After all, one of the other advantages of mid-battle leveling is that it does not provide your opponent any chance of regeneration.

Or, it might mean exploring some amount, then using a much higher attack power such that you still have dealt significantly more damage to the opponent than what it heals for exploration. Monsters heal 1 health per level per tile, just as you do. Setting up exploration-involved fights means having at least that much attack power above theirs. For example, if you want to use a 3-tile slot to gain just enough, you want to have at least a 30 point advantage over a boss so that you don't lose everything that you just did. To adopt this towards mid-battle leveling, you don't need to compare it to the enemy's attack power, and simply just need it so that the health you need to survive a mid-battle level won't heal the boss such that your damage is undone. The higher above their healing rate you can make your damage, the better.

You don't have to have revealed the whole map, but you do have to be very careful about where you are moving. You need to have one or more monsters already revealed that would allow you to level up by defeating them. Since you get first strike against any monster lower level than you, you should be able to defeat any monster (that doesn't have first strike itself) even at 1 HP as long as you do enough damage to kill it in one attack. Whatever you do, don't reveal any new squares on the map until you are prepared to have all monsters heal. I've sometimes been able to get a monster very close to death, and then reveal one square to get the last mana or health I need to finish him off, but in general the monsters will heal too fast for this to work.

Of course, if you have APHEELSIK, that changes everything, as you can poison a monster and walk around to heal yourself without healing the monster, provided that you don't attack anything else.