The formula setup you'll find here is absolutely, without a doubt wrong! I warned you. These are only presented in a mathematical sense out of a really poor sense of aesthetic.
The chance to land a debuff depends on several things with slight variations between them. From personal observations, the chance to land a debuff depends on:
One of the base stats affects the chance of resisting a debuff
CON resists stun and bleed
MEN affects poison, root, sleep resistance and basically everything not covered by another stat.
From experience, base stats do not provide a great amount of resistance to a debuff. There are no exact numbers to provide but you're looking at a 1% or even less difference per point of a base stat in debuff resistance.
Magic-based debuffs are affected by both the M.Atk. of the caster and the M.Def. of the target.
The level of the skill affects the chance to land a debuff. I'm calling this Learn Level on L2P. Basically to ensure that a debuff will land, you need to keep the skill leveled up. In general, getting a debuff skill at level 70 will ensure that it lands normally on a target who is level 70, but has a reduced chance on a level 71 or higher and of course an increased chance on a level 69 or lower target. Specifically, it sets an effect multiplier which seems to correlate to a target's level similarly to accuracy & evasion. Skills like shield stun have an extremely high effect multiplier and skills like a tyrant's stun skill have a low multiplier.
Any buff a character might have to resist a specific debuff, such as Resist Shock, Invigor, Mental Aegis, etc.
Since little is known about the actual formula for determining debuffs, here is a general outline of how it seems to work:
Resistance would be anything like Armor Set Bonuses, Buffs, Passives, Toggles or such that resistance the debuff in question. There are many different ways to resist various debuffs.
Stun - (Base Land Rate/CON Modifier*Resistance)*(Learn Level/Target Level)
Physical Bleed - (Base Land Rate/*CON Modifier*Resistance)*(Learn Level/Target Level), can be cured by a cure bleed spell.
Magical Bleed - (Base Land Rate/Men Modifier*Resistance)*(Learn Level/Target Level)*(M.Atk./M.Def.), cannot be cured.
Poison - (Base Land Rate/Men Modifier*Resistance)*(Learn Level/Target Level)*(M.Atk/M.Def.)
Viper Cubic ignores any kind of resistance to Poison because the genius who coded the skill forgot to make it poison attribute. It can be cured by Cure Poison skills.
Holy Aura - (Base Land Rate/Men Modifier*Resistance)*(Learn Level/Target Level); holy aura is not a magical skill
Root/Hold - (Base Land Rate*Men Modifier*Resistance)*(Learn Level/Target Level)*(M.Atk./M.Def.)
Physical Slow (Cripple) - (Base Land Rate/Men Modifier)*(Learn Level/Target Level)
Magical Slow - (Base Land Rate/Men Modifier)*(Learn Level/Target Level)*(M.Atk/M.Def.)
Sleep - (Base Land Rate/Men Modifier*Resistance)*(Learn Level/Target Level)*(M.Atk./M.Def.)
Paralyze (Base Land Rate/Men Modifier*Resistance)*(Learn Level/Target Level)*(M.Atk./M.Def.)
Misc. - most of the rest like the overlord debuffs are the same as roots
