Tainted Grail: Conquest

TL;DR
Use Zealot
Get Quick Aim Passive
Try and reduce deck to 8 cards , 4 of which are Feints
OR Use deck > 8, stacked with Blue cards that give Armour/Barrier
Get to final boss, spam Blues to stack damage buff.
Once you are ready to take attempt (either buff > 5000% OR over 2500% AND enemy Armour < -100), use items, then Ultimate, fire Spirit Arrow at boss (CHECK FIRST, YOU CAN SEE ON THE CARD WHEN YOU AIM HOW MUCH IT WILL DEAL!)
Win

This will require the use of the Zealot class and during the course of your run, you will need to be somewhat lucky with the RNG. This solution is also combined with the "I Don't Need Much" achievement, for beating a run with a deck of 8 cards or less and the "Zealot" achievement, for achieveing level 20 using the Zealot class.

The Zealot starting deck is 14 cards in size, so you will need to remove at least six of these if you want to earn both of the achievements at once, as well as to use the strategy to its maximum ability (though it will work with a deck size greater than 8, it has to be built correctly). You want to remove (and never take) any Quiver (Orange) cards, as these only hold a Zealot back. You also want to remove the Maneuver (Blue) cards "Bow Draw" and "Stepping Stones", as well as the Attack (Red) cards "Smoke Shot" and one of your standard "Shot" cards. If you are using a deck greater than 8 cards in size, take additonal Maneuver cards that grant you Armor and/or Barrier. You can also swap out one or more of your Shot cards for something like Muscle Tearing Shot (400% damage but draw 2 less cards next turn), which is what I used for gauging the amount of damage my Spirit Arrow was going to deal.

The opportunities for removing cards are few and far between, so even if they will put you in a bit of a dicey spot, take the opportunity when you can. To my knowledge, the ways you can remove cards are:

Witch - Sacrifice 80% current max HP and remove up to 5 cards
Elderly Archer - Remove 2 cards at no detriment if his quest has been completed. Otherwise costs 10HP (I think)
Hunting Grounds - Instead of choosing to hunt, rest and fight the monsters that come, then choose to remove up to 2 cards.
Master-at-Arms - The Master-at-Arms villager can remove 1 (by default) card per run for 100 Wealth, can gain another if has the upgrade costing 400 Card Shreds (1 gained per card not taken when opportunity arises)
Guardian Kill - You may remove 1 card after each Guardian

This means that you will need to use the Witch in order to pare your deck down small enough to be able to earn the 8 cards or less achievement.

The other things to look out for or upgrade during your run are default/starting Armour (the more you have, the better this works) and the opportunity to take the Passive Skill "Quick Aim". This is how you will be able to deal the required level of damage, as it gives you a 2% damage buff, per point of Barrier remaining at the start of your next turn. This is absolutely essential for this to work, grab it when it appears. Also try and get as many of the following items as you can:, making sure to have them usable in combat BEFORE triggering the final battle

Acid Flask - Lower Armour by 100 on single target for 2 turns
Acid Bomb - Lower Armour by 50 on all enemies for 2 turns
Consecrated Oil - Deal 100% damage more for x turns (default 1)
Berserker Potion - Increase damage by 100%, lose 50 Armour

Once you are ready, trigger the fight and spam as many Blues as you can every turn. The more Armour and therfore unused Barrier you generate, the faster this goes. I buffed myself somewhere around the 6500% mark, but this was way, way too much, you probably only need 4-5000%, or if you have Acid Flasks or Bombs, even as low as 2500%. Once you are at a comfortable level, use all your items, then your Ultimate ability at full charge. The Spirit Arrow it generates is already at 400% damage, and will deal even more with negative Armour, so you should easily clear 3000 damage. On my run, with a 6500% buff, the boss on negative 180 Armour and a quad damage Light Arrow, I dealt around 16500 damage, which is why you probably don't need to buff nearly as much as I did, but I wanted to be safe.

Use pathfinder.
Just keep up insight double blocks.
Try to get block cards that buff for next turn.
Every turn your damage increases from focus passive as long as no hits get through to you.

Grab 1 super hard hitting card early and the rest are low cost and block support cards.

I never got hit in the 2nd or 3rd area and 1 shot both bosses.

Focus on being able to get through blue stance cards fast. Build your ultimate and keep drawing blocks. After so many turns you will oneshot anything.

Good cards.

--Deceitful pummel.
0 cost. Brings 2 cards back from your graveyard. (I encountered a glitch. Had to pause and unpause on xbox before i could confirm with Y button)

--Deep breath / double block
1 cost. Both buff next turns plays.

--Refreshing Breeze
0 cost. +1 energy -25% damage this turn.
Good for making extra blocks and building ultimate.

--Reposition
0 cost. Swap your hand for 4 new cards.
Good for building ultimate and finding blocks and such.

--Calm Breeze
2 cost. Paired with insight it is 3 free energy at the beginning of a turn.

--Skullcracker / power blow.
These are the kill shot cards i hit 5k with after powering up. Only need 1 each in a deck because you can rotate cards out fast.

Difficulty set to easy
Use Zealot class
Get Quick Aim Passive
Reduce deck to 8 cards , 4 of which are Feints

The deck I used was 8 cards total
- arrows
aimed shot x1
smoke shot x1

- maneuvers
rancid battlefield x1
tails x1
feints x4

Win every fight by spamming Feints

for this achievement spamming Feints until you can use Ultimate, or Aimed Shot to kill with over the 3000 damage. you can use card rancid battlefield (-20 armor/ flip -30 Armor) to decrease the targets armor. can be done on any enemy, the weaker the enemy the easier the achievement.

Guardian Kills -
for the Guardian fights in each area remove a card or grab a skill as needed.

don't grab any cards except for your deck, and remove cards as fast as you can before you grab as many skills as you can while getting XP and runes for armor and skills to help your feints. block ever hit with barrier from your feints.