There are multiple ways you can approach this achievement.
Sadly for us, success of an attempt is fundamentally reliant on a lot of RNG.

Unfortunately I don't know whether usage of Sorcery Shift ritual affects this achievement. If it doesn't, there are quite a few strategies you could employ to get it. After all, having two available spells is way better than having one.

But until I have a chance to confirm that either way, I can point you to things that work right now.

Things that you will need no matter which approach you choose:

• Yes, there is no way around it (there is), you will have to find Everguard (Gain 1 Tenacity)
  
• More dice. The best choices are Amulet of Sacrifice (+1 Dice, +1 Sap), Dreadstone (+1 Dice, 4 Damage on enemy death), Ley Line Focus (+1 Dice, enemies get 1 Power). Fire stick is also fine, 2 damage don't really make a difference.
  
• Talisman of Suspension (2 dice persist between rounds). This can be optional if you feel the heart of dice, for everybody else -- this is a way to smooth out rolls between rounds. Roll ones in a round? No problem, you saved a 6 from the previous one.

Optional, but really nice to have:

• Exalted Medallion (All dice +1 value in Countdown slots). Gives you slightly more leeway and allows to cast Stone Meld with 1 dice.
  
• Horn of Virtue (1 blessing when getting 10+ Block from a spell). Protects you from rolling 4 ones and eating dirt.
  
• Runic Infuser (-3 Countdown each turn)
  
• Protective Feather (Gain 2 Barrier turn 1). It adds up.
  
• Cube of Darkness. Funny and can be decent if it rolls a 5-6.

For rituals you want to have

• Soul Grasp. Obviously.
  
• Conjuration Chorus (get echo for the next cast) is very good, but the cooldown is noticeable. Flask of Echoes fills the same role.

Bosses: I feel like it is better to go for this when you roll Arkanax as the final boss. Umbraxis is also possible, but it's more unpredictable and hard when you don't have many tools available. Of course, by the time you arrive to the third map, you already know whether you have a shot.

Probably the most obvious and boring way that comes to mind when you read the description.

"What if I just get come thorns and then play for a couple hours each day casting Stone Meld until the boss dies from old age?" Well, being boring is not a crime and this approach works.

What you will need:
- Stone Meld
- At least one source of thorns. Of course having multiple is much better. Among them:

• Thorned Pendant (Turn 1: Gain 4 Thorns)
  
• Needle rune (Depleted: Gain 2 Thorns)
  
• Spike seed potion (Gain 5 Thorns)
  - And last but not least, you will need to kickstart your blocking spree. Ironskin potions are very good for that. Another option is Enchanted Cloak (Get 15 armor turn 1). Finally, you could use a ritual to do this, but you might not survive until then.

The gameplan is as simple as it gets. Cast meld every turn and wait until the boss kills itself with Thorns. Good luck and bring a book with you.

Now this is where the fun begins.
This time the fight will go much quicker, but it is also a bit harder to assemble the pieces.

What you will need:
- Maul. I recommend going with the right facet.
- This time actual damage numbers are important, so you will need to stack it using:

• Rage Blade (-1 Dice, +15 Fury each turn). Arguably the mvp in this build.Allows to hang in until the power really starts making a difference.
  
• Damned Shield (+1 Power +1 Sap each turn)

What you will want:
- Might rune (+3 Damage) is a nice to have
- Darkheart (Mark enemies for each four 4's rolled)
- Portentum (Apply Portent each turn). Absolutely a dream pickup for this build, but as usual, it's up to RNG to decide.
- Tidebreaker (Block is converted to Fury at the end of round). Another dream pickup that I didn't get a chance to try out, but it allows to convert block into damage and back each turn.
- Crystal Snort potion (+5 Power)

There are two additional choices of ritual in this scenario. Empower and Arcane Infusion. Both are quite slow without additional dice rolls (or kills), but it's still a power boost.

The plan is to cast Maul once per round to make use of Rage Blade and stack up armor. Don't use it more than once per round until you get a significant amount of power (35+). Always have at least 5-6 total value in dice saved via Talisman of Suspension so that you can recover from bad rolls (if you roll garbage twice in a row, it's likely over unless you saved up a lot of block. Runic Infuser is a good insurance against this. So is a potion bottle full of dice).

I went with this for my attempt, and here is the result. As you can see, I didn't have everything I would have liked to, most notably -- no Soul Grasp, but it worked out in the end.

For people who REALLY feel the heart of the dice, there is a spin on the defensive plan that I can offer.
There is an event that allows you to pick a spell from another character. And if you are playing Lapis, and you roll Stone Meld from this event, you have an opportunity to do something really funny.

In addition to everything listed in necessities you will need:
- Poison Core (5 Poison to enemies turn 1)
- Serpent's Kiss (1 Poison to a random enemy when getting 5+ Block)
- Ring of Poison (Poison applied to enemies is increased by 1)

If it good? It's better than thorns if that's what you are asking.
Is getting all the pieces realistic? No.
Should you send me a screenshot if you actually pull this off? Yes.

Good luck!