A battle reduction (BC), where basic enemies have a chance to spawn as different enemies (X% Fast, X% Tank, X% Ranged, X% Boss, X% Protector).
A lab upgrade also exist with the same name, which reduces the effect of the battle condition by a percentage (up to 10 levels = 10 %). This lab is part of the Group 3 BC Labs. You unlock this lab in Milestones by reaching Tier 19 wave 500.
Basic's Ultimate - Lab values, costs, and duration (10 levels)
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For all BCs, see: Battle Conditions.
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