Balrum

Do NOT waste points in raising your VIT stat, you can get this achievement with other stuff.

1 VIT = 50HP
You get 50 HP on level up.
Old Shrines can give you extra HP, they only work a few times across all shrines, so you might find extra shrines greyed out, this is normal.
In Kyler's Village, you can pay the priest with Thaler only to get small HP boosts.
Rotting Voodoo Doll (+10 VIT for 20 turns)
Amulet of Life (+3 VIT)
Gold Ring (+1 VIT)
There should be a Belt for your class that gives at least (+1 VIT)
Eating a +8 VIT food using Cooking L4
Special Effects like Slept Well from resting in a bed (+2 VIT) and Good Deed from Raven@Erdal (+2 VIT)

10 levels = 500HP
Father Hein Blessings = 90HP
Male Old Shrine = 20x2+30x2 = 100HP
Rings/Amulet/Belt = 250HP
Food = 400HP
Slept Well Effect = 100HP
Good Deed Effect = 100HP (2 bread and 4 apples @ Raven)
Trinkets = 500HP
Total = +1090HP

You can add more by wearing VIT boosting equipment, so spending points in VIT at the start or during level up is a bad choice, I got this at Level 8.

If you are still having trouble, get Craft Armour 4 and get VIT on ALL your equipment, you can reload after you get it.

You should ignore VIT and WIS at the start of the game and start your build with 2 or 3 INT.

1 INT = 50MP
Trinket: Tablet of Sacrifice (+5 INT for 20 turns)
Moon Amulet (+2 INT)
Ring of the River x 3 (+1 INT each)
Ring of Sacrifice (+1 INT)
Speed Potion Effect (+2 INT)

You can make a save then try go for this achievement by:
Save before leveling up, when you level, choose Intelligence.
Training INT up to 3 times then wear INT boosting gear.
When you reach Base INT 5, use the Intelligence Book in Darkwood to get +1 base INT.
Training MP up to 4 times.
Eating a +8 INT food using Cooking L4
You can learn create armor 4 to create INT equipment, but you'll be fine with the gear you have hoarded.

Training INT 3 times @ Lia (9~11LP) + Book Apparatus = 200MP
Training MP 4 times @ Lia (4LP) = 100MP
Female Old Shrine = 20x2+30x2 = 100MP
Rings/Amulet = 300MP
Food = 400MP
Speed Potion = 100MP
Trinkets = 250MP
Total = 1450MP

Add your starting mana to the total above + any INT from your armour and you should be set.

A crit gives 200% damage, with a potion buff, this becomes 300%.
So your goal is to do around 350 damage with an attack.

Mages have it easier since their ultimate spell does up to 600dmg and a spell crit should give you this achievement. You can also find/craft +Fire damage items to get this, but it shouldnt be necessary.

Preparation Scroll will guarantee a crit in your next activated ability.

Check this guide for info on weapons.

steamcommunity.com/sharedfiles/filedetails/?id=2517074814

Black Knight Greataxe (410) / Haste Dagger (290) / Black Bow (240)

*You dont need those specific weapons, but something almost as good should work.

Extra Boosters
15% more crit damage from Ring of Thorns
Extra Crit Damage Potion (300% crit damage) *This is key
30 Phys Dmg from Brooch of Negligence
2x4 Phys from 2 Blood Rings
Eating +8 Stat Food

Bow Only:
230 avg dmg from Black Bow.
25 avg dmg from Steel Arrow.
Close Shot = +70 dmg Avg.
11 Base DEX + 24 DEX (my archer build) = 49 damage
If you already do more than 350dmg, you can use Perfect shot instead.

With a DEX Dagger
Haste Dagger Avg = 270
Precise Cut = 60 avg dmg *More because it ignores armour

With Swordplay 1 and using book to L2:
You can reload after getting the achievement to save your LPs.
No Mercy Skill = 210 dmg avg
Anger Strike = 125 dmg avg.
Shatter Armour = use this up to 3 times, to do even more dmg.

This is a video of how I got the achievement:

In NW Balrum, on your way from where the teleporter drops you to the Hunter Village, you will find this trapdoor, inside, something invisible is moving, check the footsteps and interact with it to get the achievement.

You will need STR to wear the best armours, if you are a STR build, this is in the bag.

Every 6 points in STR (doesnt have to be base), gives you 5 armour.
Key: Item Name(Arm)(STR)*

No requirements:
With a Bear Hide Potion you get (35)
Medal of the Guardian (7) (1)
Cold Ring (4)
Crimson Ring/Servant of Adacus, 1ea (3)(1)
Laws of Adacus Trinket (50!)
+8 STR Food Effect (0)(8)
Good Deed Effect (0)(1)
Totals = (99)(13)

DEX build:
Griffin Armor (22)(3)
Griffin Cap (10)(1)
Griffin Leggings (13)(1)
Griffin Gloves (6)(1)
Cow Leather Belt (5)(1)
Shoes of the Sands (8)(2)
Totals:(64)(9)+(99)(12)=(163)(21/6=15A)=178 [Need 12 more]

*Shields grant extra armour but you need base STR.
The weakest shield gives (8) Req. 3STR.
Check the equipment encyclopedia.

Strategy for non-warriors:
Hard Save before leveling up.
Choose STR when leveling.
Go to Torric at Balrum SE, train STR to 5 STR.
Use STR table at Darkwood SE when you have 5 STR to get 6.
If you cant get to STR 6, STR 4 is fine but you need to have saved some equipment like the one I list below then wear/do everything in the no requirements section.
Pray for the best.

I finally got it with:
Griffin Cap
Veteran Armour
Warrior's Plate Gloves
Warrior’s Plate Leggings
Cow Leather Belt
Shoes of the Sands
Warrior's Shield
Light Sword

DEX build with 6 STR
Replace your Griffin Armor with Plate of the Gladiator(31)(3) [+7A]
Use Heavy Shield(18)
That would do it.

If you are a mage, you should go with STR 6, wear Warrior Equipment and just spare yourself the suffering, save every high armour gear you find with STR 6 requirement or below.