If Chimeric Staff is already an artifact creature when you activate its ability, Chimeric Staff stays an artifact creature but the ability resets its creature types. You always apply the effect from this card before counters or other effects that modify power and toughness. For example, if you pay {3} to make it a 3/3, then someone gives it -1/-1, it will be a 2/2. If you then activate the ability with {4}, you will apply the "4/4" effect before the -1/-1 effect, so the end result will be a 3/3. Damage that was dealt to the creature stays on it.