The shield amount will stack when multiple monsters are equipped with shield rune (225% HP at Max if 5 monsters use 3 sets each). This shield is removable buff. The shield will reactivate when a new wave starts (Secret Dungeons excluded), but does not reactivate after revival.

Do Shield sets stack SW?

If multiple Champions on the same team have Shield Sets, the value of the [Shield] buff from the Artifact Sets will stack. For example, if two Champions with 6,000 HP equip Shield Sets, the whole team will gain Shields worth 4,000 HP (2,000 + 2,000).

Where do I get shield runes?

Rune Shield Location: Where to Find Rune Shield

Defeat Black Phantom Ostrava in The King's Tower (1 - 4). He drops the Rune Sword and the Rune Shield.