It took several runs to get to 29. Don't remember the exact number, but not that many. I was actually surprised that I got that far, The arena bubble was quite small for at least 10 last waves so I guess there was not that many enemies to deal with. I probably got lucky. Will try this other hinted dominant strategy :)
Hit response doesn't need to shake the screen. A particle effect or a color flash and a subtle sound may be sufficient. Currently it looks like there is nothing.