for these type of one off's.
I typically add the FX in the FX bin and automate, edit it how I want it, then bounce it.
as mentioned above, there are a few ways of going about this. my advise is to try a few and go with what you find easiest, most productive.
may I make a suggestion? while destructive editing, always leave yourself the original/unprocessed clip.
this way if you bone head something, you always have your original clip to fall back on.
you may even find freezing the track an option as well. freezing will free up resources while still leaving you the original with its FX to make changes on the fly if needed by simply freezing/unfreezing.