Dicey Dungeons

These are the methods known so far:

1.
Simply find an enemy with Thornes
Have a Limit Break ready
Let the enemy build up a few stacks of Thornes
Use the first Limit Break
Attack the enemy a few times until you charge up the Limit Break
Use the second Limit Break

2.
Select the Witch
Try and get the Crucible spell
It lets you reroll your dice to a new Burned dice
Use the first Limit Break
Then you can just reroll that Burned dice to a new Burned dice
repeat until you charge up your second Limit Break
Use the second Limit Break

3.
Select the Warrior
Play Episode 3
Collect the Boomerang (DO NOT UPGRADE FOR THIS ACHIEVMENT)
Collect a Limit Break
Use the Fury on the Boomerang, preferable using a die of 4 or higher
This should deal 8-12 damage to you, allowing you to use limit break again.

I recommend doing this with the Thief on episode 3. Mainly because of the ability to know what dice you roll each time.

To make it easier you can do these things:

Upgrade the Dagger.
This makes it so instead of needing a minimum of 7 dice, you now need a minimum of 5 dice.

Pick up Hall of Mirror.
This makes it so you get more dice, which is not necessarily required because of the Limit Break you get in episode 3.

Pick up Counterfeit
Same as Hall of Mirror, it's not necessary, but it makes it easier.

The simplest way in my opinion to achieve both these achievements is to make a shield build with the Warrior.

The equipment you are going to need is:
Iron Shield
Shield Bash
Potentionally Healing Crystal

All you need to do is to focus just on making shields with Iron Shield
When you have 40 shields you can use Shield Bash to get one of the two.

To kill the boss with full health you do the same, heal with Healing Crystal when you take damage.
To make the fight easier, you might want to look for Nightstick, Counter Spell or Snowball.
Those three are very good equipment to minimize the potential damage taken, which makes the shield build better.