Tainted Grail: Conquest

TL;DR
Get Quick Aim Passive
Try and reduce deck to 8 cards , 4 of which are Feints
Win every fight by spamming Feints until either you use Ultimate, or Aimed Shot to kill a target in one shot.
Repeat as necessary

The Zealot class is part of the Watchers of Tuathan faction, so fight using low cost Arrow (Red) cards to deal damage, Quiver (Orange) cards to draw more arrows in exchange for Ultimate charges and Maneuver (Blue) defensive and buff cards. Blue cards will also have one or more extra states that they will switch between every time the card is used (e.g. Feints side 1 gives 10 Armour this turn and 100% of Armour as Barrier, Feints side 2 (called Unbreakable Feints) gives 150% Armour as Barrier). Cards stay flipped between battles, so you can save powerful uses for when you need them, or can avoid bad effects by using them before ending a fight. The Zealot Ultimate summons a 0 Cost Spirit Arrow, which will be randomly chosen from the available pool. Once you have used it that fight, it will be removed from the pool until you have used them all or the battle ends. An important difference to know is that Barrier only lasts one turn for any of the classes in this faction (Zealot, Sentinel and Apostate), any remaining just disappears when you begin your next turn.

In order to easily reach level 20, you want to try and do 3 things.

1. Get the Quick Aim passive skill, which makes every fight trivial when combined with your Aimed Shot, Muscle Tearing Shot or UIltimate
2. Give youself as many Armour buffs as you can. More Armour means more Barrier from using Feints, which means bigger damage buffs and safer HP
3. Remove all Quiver Cards, Bow Draw, Stepping Stones, Smoke Shot and one or more standard Shot cards. A smaller deck means pulling one or more Feints guaranteed every single turn.

The Zealot starting deck is 14 cards in size, and if you remove all the cards listed above, you should have a deck of 8 (which will earn you the I Don't Need Much achievment if you beat the final boss).

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 - After each Guardian you can choose to remove 1 card

This means that you will need to use the Witch in order to pare your deck down small enough.

For the early fights, you should be able to get by with minimal usage of Feints and can spam Aimed Shot and other Arrows like you would with a Sentinel.

Once you have Quick Aim, every fight is as follows:
Spam Feints/Unbreakable Feints until damage buff big enough to kill an enemy in one Aimed Shot or Ultimate Arrow
Use Aimed Shot/Ultimate, killing enemy
Repeat