It seems like sometimes when my companions kills an enemy, i dont get xp, do i need to get the first or last shot? Or just hurt them?

I think you need only to hurt them. At least, I sometimes get random XP if I had few minutes earlier wounded a molerat who then ran away.

I think it depends on the percentage of damage you inflict vs. the percentage of damage your companion inflicts.

Thansk for the answers.

My subconscious brings the number "33%" to my mind from discussions a few years back.

If you do at least 30% of the damage then you will get some exp. If you do more than 50% then you get full exp.