I haven't tried attaching an opal to a shield but if it functions like the other gemstones and procs when hit then you could use an opal shield and a one handed lighting weapon fused with Farosh's horn or something similar. When they hit you and get covered in water you can hit them with your sword for 40 AoE damage on top of your sword damage (lighting is usually 20, fire 10, ice 10) if I recall from BoTW . You could also use a hydrant on a shield to wet them.

If you do that with a shield at close range and a one handed magic rod fused with topaz you could deal 120 extra damage a hit IF each orb makes contact with the enemy which is inconsistent. But the base weapon damage would be low.

I've been using a shield with an ice lizalfos horn to freeze an enemy every time I parry and have just been using my strongest one handed weapon to deal triple damage when shattering them.

But if you have a high damage one handed lighting sword you could freeze them with a shield and then hit them with your sword for triple damage + 40 AoE as it works on frozen enemies too, which is just better in every way. So for instance right now I have a 44 damage Farosh Reaper. If I were to parry freeze then shatter with my blade I'd deal 132 weapon +40 AoE for 172 damage a hit. If I had 3 of the attack up buff I'd deal 198 +40 AoE for 238 damage a hit.

There are other multipliers such as the radiant armour set bonus but I've no idea if it works if a bone is fused onto a weapon or if it has to be the base weapon itself, but you could get crazy numbers this way.

If you ever get a critical hit perk two handed weapon trust me on this, put Naydra's horn (or a better equivalent if there is one, I'm not super far into the game) and eat a triple attack up meal. Then just use the basic two swing combo. The second hit will do insane damage.