Grab some boxes, stack them for Height advantage.
Drop bomb on boxes and throw them at the ground between both Shadows.
This should be enough damage to break the shield.
Throwing bombs from the ground with height advantage will cause your damage to hit twice, 1 time from explosion and 1 time from height advantage. Doing so will save you on amount of explosives you need to bring.
Quick example on Explorer 7 smokepowder bombs were used.
5 would be bare minimum with some randomness to the damage, 6 should be more than enough.
Tactician I would 8-9 bombs.