How is "Bonus Slag damage" calculated for Slag-Licked?

I noticed I was gaining progress toward Slag Licked while using a Slag SMG:

Slag-Licked: Deal some bonus damage to the slagged enemies (5000).

Slag does bonus damage to enemies when hit by other weapons but I generally don't swap from my slag gun to deal bonus damage; I let teammates shoot slagged enemies and I either keep firing or slag more enemies.

Does damage dealt from slag weapons count as "bonus slag" damage or am I getting a bonus from my friends dealing damage to slagged targets or what?

Solution 1:

You are the only one who can add to your challenges, save for one or two specific ones like the car compacter in the dust. So your friends hitting enemies you've slagged doesn't add to the bonus slag damage the challenge has said you've dealt (although it does add to your friends challeneges, since they're the ones dealing the bonus damage), and slag weapons don't deal any bonus damage, only their own damage.

As someone has said you do indeed need to switch to another weapon after slagging an enemy to deal bonus damage. It's also possible to use slag explosive barrels or, since you seem to be playing as the Siren, using the ruin or scorn skills.