how do run's work?

[Horadrim]

am not quite sure how runs work if a mob is week to fire do i use fire run or the run opasit of it?

How do "Runes" work.

I'm going to use fake numbers, you'll have to refer to the wiki to find out what the real numbers are -- these are just for an example:

When you activate a Rune, you gain affinity with that rune -- you deal damage of the same type as that rune and take less damage from the element it is strong against.

Ignis (Fire) gives you +10 Ice Resistance and EnFire, for example. If the monster you're facing is weak against fire, you want to use Ignis. If it is a fire monster, however, you want to use Water (as per the last sentence, you gain +10 Fire resistance because Fire is weak against Water).

Basically memorize the elemental relationships:

Fire melts Ice -> Ice chills Wind -> Wind moves Earth -> Earth grounds Thunder -> Thunder shocks Water ->Water douses Fire

Light and Dark have a mutual relationship, whichever one is stronger beats the other:
Darkness consumes Light | Light penetrates Darkness.

Runes go to the right (what you'll hurt more)

Rune : Fire > Ice > Wind > Earth > Thunder > Water > Fire | Light <-> Dark