How to effectively use explosive weapons against metal shields?

As far as I can tell there are three things you can do with explosive weapons when fighting against an enemy with handheld metal shield(like nomads).

1. Shoot the shield directly
2. Shoot the nearby terrain to damage the enemy with splash damage
3. Shoot the enemy directly after flanking him

Basically what I want to know is which one does more damage than which or which ones does equal dmg to which etc. I want to know what the best option is.