Prohibitively expensive real-world invisibility tech that defeats the base's anti-magic charms entirely.
The Madness Monkeywrenchers infiltrate The Castle grounds using invisibility that is not magic at all. The base is covered in anti-magic charms, but the monkeywrenchers' concealment is a tech-based cloak — something that exists in the real world but is considered prohibitively expensive for all but the richest of soldiers. The cloaks are high quality, and the anti-magic bubbles simply do not detect them. Even the veterans were not anticipating this.
The workaround is optical rather than magical: Donut's sunglasses on the heat setting can pick the cloaked mercs out, slowly, and a handful of other defenders have similar abilities. Once a cloaked wearer takes damage — Carl's Banger Sphere draws a blood splatter — the invisibility stutters and the outline becomes clear enough for crossbowmen to finish. Tina can apparently find them too.
Invisible in operation; the effect stutters and reveals a wearer's outline once they take damage.
The attackers' key advantage — tech invisibility that magical countermeasures cannot see.