Third, I generally use the damage numbers on page 42 of the DMG with the following guidelines:
- Is it clearly a one-time thing (“I leap onto one end of the broken tree, angling the other end up towards the charging monster like a pointy spear”) that’s unique to this fight? Be sure to use the limited damage expressions. If it’s something that could be repeated every fight (“I shoot the monster in the eye with my arrow”), I use the regular expressions.
- The first time in a fight an improvised attack is used by a player, I raise the damage expression one notch higher than I would normally, just to reward the player for thinking outside the box.
- If the improvised attack is just doing damage, I lean towards the highest damage expression (whether normal or limited), then I lower the damage one or two notches depending on the effectiveness of the special effect that the player wants to add.
- I like the idea of making each unique trick into an encounter power – so that a person can try multiple improvised stunts, but each one only once. I’d allow a stunt to be used more than once in an encounter, honestly, but I’d mitigate the special effect and dial the damage down one notch each time to discourage this.
For instance, using the aforementioned examples for 6th level PCs:
“I leap onto one end of the broken tree, angling the other end up towards the charging monster like a pointy spear”: Limited damage expression, as this is both cinematic and highly situational. 3d10+4—reduced to 3d8+4 if the PC wants the tree trunk to immobilize the monster.
“I shoot the monster in the eye with my arrow to blind it”: This could happen every fight. Normal damage expression, start high and reduce by two levels because blind (whether save ends or end of your next turn) are both really good. A mere 1d6+4 damage plus the special effect, or bumped up to 1d10+4 if it’s that person’s first improvised attack of the encounter.
Don’t sweat the numbers too much. 4e monsters have enough hit points that it’s better to err on the side of more damage if you’re trying to reward creativity.