can you post a screenshot of the mission requirements?

Also could you post a screenshot of your champs as well? Thanks. 

I am asuming you need any champ that has remove buff, steal buff, remove shield skill.

so one of the following.

REMOVE BUFF (AOE)

Legendary: Astralon, Elegaius, Gurgoh the Augur, Guurda Bogbrew, Lady Kimi, Prince Kymar, Rae, Ramantu Drakesblood, Sethallia, Suzerain Katonn, Underpriest Brogni, Venus, Vergumkaar, Yannica.

Epic: Chancellor Yasmin, Chonoru, Grimskin, Hope, Madame Serris, Rian the Conjurer, Seer, Sikara, Valla.

Rare: Banshee, Grinner, Hospitaller, Skirmisher.

REMOVE BUFF (TARGET)

Legendary: Aleksandr the Sharpshooter, Cruetraxa, Rhazin Scarhide, Roshcard the Tower, Shamrock, Sigmund the Highshield, Tormin the Cold.

Epic: Alaric the Hooded, Baroth the Bloodsoaked, Geomancer, Gorgorab, Juliana, Luria, Shaman, Snorting Thug, Spider.

Rare: Candleguard, Chopper, Conquerer, Courtier, Hurler, Ogryn Jailer, Quaestor, Solaris, Totem.

Uncommon: Battle Sister, Skinner. 

STEAL BUFF (AOE)

Legendary: Samar Gemcursed, Skartorsis.

Epic: Madame Serris, Skathix.

STEAL BUFF (TARGET)

Legendary: Basileus Roanas, Elder Skarg, Errol, Gomlok Skyhide, Harvest Jack, Kantra the Cyclone, Konstantin The Dayborn, Ma’Shalled, Shamrock, Teela Goremane, Tormin the Cold, Warchief.

Epic: Alaric the Hooded, Aothar, Fodbor the Bard, Genbo the Dishonored, Geomancer, Oboro, Peydma, Snorting Thug, Torturehelm, Warcaster.

Rare: Anointed, Candleguard, Hellfreak, Honor Guard, Metalshaper, Mystic Hand, Ogryn Jailer, Slitherbrute, Temptress.

You will need to check the skills as some of them only remove specific buffs. For example I know Sigmund the Highshield removes shield buffs, but Genbo the Dishonored steals increase attack buffs and has a chance to steal another random buff.

So as long as you have one of these and enough accuracy (note, again check the skills as some of them don't need accuracy to be successful) then you should remove any shields they have.

I think this mission (which is actually magic keep, not void) can be beaten just by hitting the shield hard enough to break through it. You don't need any specific skills, just enough damage.