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The best character to do this with is Defect (blue).
The best boss to do this against is Slime Boss (Act 1 boss).

Slime boss does not attack on first and second turns, so you can deplete its HP to around 51-55% in the first two turns.
Now the trick: Focus on lighting orbs, have at least 3 of them up and do not be afraid of using any orb slot potion or focus potion.

At the end of turn 2, before the Slime boss would attack, your 3 (or more) light orbs will damage it by minimum 9HP, this way the Slime Boss will not attack, but will be divided to two smaller slimes.
Now the smaller slimes have the same effect, but they will attack in the second turn after being spawned, so you have to be faster to damage them to below 50%, so that they divide to small slimes.
Then finish off the small slimes (they supposed to have around 15-20HP each, the less the better)