Lately, I've been asked about my Deadnaut tactics for extreme difficulty level. Response turned out to be quite a guide (also, exceeding maximum limit of 20 000 characters for personal message), so I decided to post it here, for everyone to benefit - here is a link to the guide on official Deadnaut forums (I've tried to post it here natively, but apparently, GOG forum isn't suited for such long posts - it just hangs up every time I try to "post my message"):

https://forums.screwflystudios.com/index.php/topic,889.0.html

Hope it helps someone,
/Estel