If you see a high level boss spawned by someone else, just hit it with a really weak unit. Most people on these forums have a unit with only one single lvl 1 girl for that specific purpose: to poke a boss, but not kill it. Maybe if there's only half an hour left on it or if it's a very hard boss with several million hit point, then try an attack with a "full team"to at least reduce its health. Otherwise, just poke it once. No matter how hard or how often you his a boss, the rewards stay the exact same.