Disclaimer: This uses a classic StS-style teaching model, not a guaranteed match for Slay the Spire 2 or current patches. Powers, relics, and keyword changes can alter real damage — verify in-game.
Formula (this page)
Start with base + strength (one strength = +1 to attack damage in many cases).
If target is vulnerable, multiply by 1.5 (unrounded; then we floor at the end).
If you are weak, multiply outgoing attack damage by 0.75 (common rounding pattern; games may differ).