The easiest and quickest seed to get the achievement
Best seed to get Watch the World Burn achievement
So to make it much more easier, I have found the perfect seed to complete this achievement.
First we will select Terra with her Pyro kit. Her Pyro kit includes the Combust spell which will refresh all tiles on fire. (The kit doesn’t affects the seed if you follow the other instructions, but its better to use Pyro)
Then we input the seed: 438373281
Afterwards we can begin our run
After our first battle me must select the Frostbolt spell, and after that, we need to set our second focus to Anima too.
After that, we will choose the upper path on the map, and after beating the fight we will be getting the Backfire spell
Then to complete the achievement we have to cast Backfire on each row, so just cycle between Backfire, Combust, and Shuffle a couple times and you will be convering the entire map, granting you the achievement.
If you miss one of the steps, you wont be getting Backfire and you will have to try again.
This is a work in progress, I will be searching for a Backfire seed for every character besides Terra and I will update the guide if i found them or if find a better seed for Terra.