Some side missions will plop you into an area where tons of enemies are fighting which makes for a great opportunity to try to get a stunt or two (or maybe even a trophy). Also, if you don't progress in the side mission to where it'll autosave, you can keep killing yourself and retrying the same part over and over again until you get the stunt you're going for.
I remember retrying the same part of a side mission over and over until I finally got the stunt.
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For stunt 20, I believe the electric cables in the historic district (around the circular buildings) are a good alternative. I know you mentioned using cables, but I'd specifically use those if someone does do that route (if you can indeed do it there, it's been a while since I played the game) because compared to the cables that are ideal for this stunt in the warren, the ones in the historic district aren't above water.
