Formulas

All results are floored (rounded down to nearest INT). Many formulas have been Datamined long time ago and as such should be taken with a grain of salt (the game is currently much harder to datamine)

HP = VIT * 4.7 + LVL * 7.4
PATK = STR * 2.1 + LVL * 5.6 + 50
MATK = MAG * 2.1 + LVL * 5.6 + 50
PDEF = VIT * 1.1 + STR * 0.5 + LVL * 5.6 + 50
MDEF = VIT * 1.1 + MAG * 0.5 + LVL * 5.6 + 50

DAMAGE FORMULA = (ATK * 1 - DEF * 0.5) * 0.4  * (SKILL POWER * (1 + % SKILL DAMAGE MODIFIERS FROM SKILL LEVELS) / 100) * (1 + % DAMAGE MODIFIERS) * (1 + TARUNDA/TARUKAJA - RAKUNDA/RAKUKAJA) * (CHARGE / CONCENTRATE: 2.25 or 2.75 w/ Templar Dragon) * (CRIT/WEAKNESS/RESIST)
10% variance in final damage. 
Critical hit multiplier is 1.5x
Weakness hit multiplier is 1.5x
Resistance hit multiplier is 0.7x

HEALING FORMULA = (MATK * (HEAL POWER * (1 + % HEALING MODIFIERS FROM SKILL LEVELS) / 100) * HEALING_CONST + MIN_HEAL_VAL) * RANDOM_HEA_VAR * (1 + % HEAL MODIFIERS)

-kaja/-kunda effects (applied at end of formula) 
Rakukaja: -0.17 = 0.83x damage taken
Tarunda: -0.2 = 0.8x damage taken
Rakukaja + Tarunda: -0.37 = 0.64x damage taken
Tarukaja: +1.2 = 1.2x damage dealt
Rakunda: +1.25 =  1.25x damage dealt
Tarukaja + Rakunda: 1.45 = 1.45x damage dealt

STATUS CHANCE = BASE STATUS CHANCE + BRAND STATUS CHANCE + STATUS CHANCE PASSIVES - ENEMY STATUS RESIST + (USER LUCK - ENEMY LUCK) * 0.5
MINIMUM STATUS CHANCE = SKILL_ADD_RATE * UNKNOWN CONSTANT
It will take the higher result of the two formulas above.

TEMP ACCURACY = BASE SKILL ACCURACY * (FUNCTION1(USER AGI - ENEMY AGI) + FUNCTION2(USER LUCK - ENEMY LUCK)) * (1 * SUKUNDA/SUKUKAJA)
FINAL ACCURACY = TEMP ACCURACY + ACC BRANDS + ACC SKILLS - EVASION BRANDS - EVASION SKILLS 
There is a minimum hit chance, and it will take either that or the accuracy depending on what's higher.
MIN ACCURACY = 100 * BASE SKILL ACCURACY * CONSTANT
CONSTANT has been datamined as being probably 0.2

LUCK DIFF = USER LUCK - ENEMY LUCK
CRIT LUK VALUE = 20 if LUCK DIFF >= 30, 
                 15 if LUCK DIFF >= 20, 
                 10 if LUCK DIFF >= 10,
                 0 if LUCK DIFF >= 0,
                 -10 if LUCK DIFF >= -256
CRIT CHANCE = CRIT LUK VALUE + BASE SKILL CRIT CHANCE + PASSIVE SKILLS/PANEL CRIT CHANCE + CRIT BRANDS - ENEMY CRIT REDUC SKILLS/PANEL + Dx2 CRIT SKILL
Crit chance is clamped to lie between 0 and 100 inclusive.
BASE SKILL CRIT chance seems to be 0 or 10 on phys skills which don’t list their crit rate in their description

Counter Chance is rolled sequentially and separately, starting from the highest tier Counter first (Death Counter>Retaliate>Counter) regardless of slot order.

Party speed is calculated first at an individual demon level then at a party level.
INDIVIDUAL DEMON SPEED = (DEMON AGILITY + SKILL AGILITY + MITAMA AGILITY) * ((SKILL SPEED PERCENT + BRAND SPEED PERCENT + PANEL PERCENT) / 100)
PARTY SPEED TOTAL = INDIVIDUAL DEMON SPEED TOTAL / PARTY COUNT (It's an average)

More On Accuracy

Functions

As written above Accuracy uses some FUNCTION 1 and 2 which respectively takes the difference between the AGI and the difference between LUK. (Notation: DIFF_AGI and DIFF_LUK)

FUNCTION1(DIFF_AGI) is:

• 102 if DIFF_AGI >= 256
• 100 if DIFF_AGI >= 40
• 98 if DIFF_AGI >= 30
• 96 if DIFF_AGI >= 20
• 94 if DIFF_AGI >= 10
• 92 if DIFF_AGI >= 0
• 88 if DIFF_AGI >= -20
• 84 if DIFF_AGI >= -40
• 80 if DIFF_AGI >= -60
• 76 if DIFF_AGI >= -256

FUNCTION1(DIFF_LUK) is:

• 13 if DIFF_LUK >= 256
• 11 if DIFF_LUK >= 30
• 9 if DIFF_LUK >= 20
• 7 if DIFF_LUK >= 10
• 5 if DIFF_LUK >= 0
• 0 if DIFF_LUK >= -30
• -5 if DIFF_LUK >= -256

In a nutshell, every 10 AGI or LUCK points are worth 2% Accuracy/Evasion on average (i.e. 1 point = 0.2%). But we have some cap on AGI and LUCK impact on Accuracy. Beyond a difference of 30 LUCK points or 60 AGI points between the attacker and defender, you will not be getting better return.

And the base Accuracy when DIFF_LUCK and DIFF_AGI are near 0 is 88 to 97, depending if DIFF_LUCK/AGI >=0.

Base Skill Accuracy

As shown in the Formula, skills have base accuracy. Normal Attack is 1.0 base accuracy (i.e 100%).

NON-PHYS SKILLS WITH IRREGULAR ACC (non 0):

• Amakusa Uprising: 1
• Barbed Spear: 1.1
• Slaughter All: 1
• Third Eye: 1
• Vengeful Frost Fist: 1

PHYS SKILLS WITH IRREGULAR ACC (non 1):

• Lunge: 0.95
• Oni-Kagura: 0.95
• Mortal Jihad: 0.95
• Mow Down: 0.9
• Herculean Strike L: 0.9
• Herculean Strike: 0.9
• Hades Blast: 0.9
• Vorpal Blade: 0.9
• Gigantomachia: 0.9
• Earthquake: 0.9
• Heat Wave: 0.85
• Megaton Press: 0.85
• Titanomachia: 0.85
• Blight: 0.9
• Rampage: 0.9
• Deathbound: 0.9
• Madness Nails: 0.9
• Myriad Arrows: 0.9
• Nihil Claw: 0.9
• Tempest Slash: 0.9
• Violent Slash: 0.95
• Beloved Slash: 0.9
• Million Stab: 0.9
• Gungnir: 1.3
• Gomorrah (J): 0.9
• Hell Claw: 0.9

Chain Effect/Passives:

• Enemy Annihilation: 0.9
• Heavenly Valor: 0.85
• Dragon Blade's chain effect: 1.0
• Terror of London: 0.9
• Yaksha Claw: 0.9
• Bloodshed: 0.9

Enemy Exclusive Skills (non 1 for phys or non 0 for elemental):

• Dragon Flare: 1
• End Realm: 1.5
• Mad Ice Sword: 1.1
• Maragidyne (Physical): 1
• Dragon Tooth: 0.8
• Megaton Press+: 1.1
• Mutilation: 1.2

Basically, phys skills get -10% accuracy for being AOE or Random Target and -5% for having increased crit rate (-15% if both). Unique skills do not necessarily follow this pattern (Ouas is 1 despite high crit, Gungnir is 1.3 etc.)