It's honestly difficult doing secret actions such as ambushes, so I don't blame you for being stumped on them. Running these sorts of PvP arena roleplays in the past, I've always handled actions requiring preparations in a way that does allow for the most basic of reactions to the action in question. Usually it's matters like ensuring the action is in the post, and having any specifics messaged to me privately.

An example of this is if, say, someone decided to poison the food of a well-known food supply area. I would usually have them ensure that they've written out their character had tampered with the food in some way, and through DMs they can tell me just what they did with it, from if they poisoned it to more specific instances such as the effects of the poison and the like.

That said, a lot of this does come from having trust in all parties that the narrative is playing a focus here. It's a bit harder to write out such scenarios in pure PvP settings without being unfair to a degree. This also comes from the assumption that people are playing their characters to form. Someone known for being honorable and forthrite wouldn't stoop to poisoning food for example, and someone who isn't known for their intelligence wouldn't just sniff the food and go "nope, something is definitely wrong with that!"

In the end though, my method is pretty imperfect and requires a degree of trust to be had. It works if you want to have a narrative focus over a competitive one, at leaat~