So here are some hints:
1. Get the watering robot occupied for a long time at a place where it doesn't see much. So then Ian can roam more freely.
2. What would be reachable by a spear from Ian's place that the robot goes to regularly? What action can you perform with a spear?
3. (Here comes the solution:)
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Poke a hole into the pot of the palm. (You can either use the spear on the hedge or on the palm, both should work.) Will only work if you wait for a time where the bot is not looking around and not looking in the general direction of this tree. Then watch the bot watering the palm in an endless loop without avail.