I have found a problem with Mission X. I have played it twice with the Genestealers, and both times I have been thwarted by two Space Marines who do nothing but sit in the back of the target room and kill every genestealer on the way in there.

My suggestion is to get rid of the corridor that attaches the target room to the rest of the board and just attach the target room directly to the rst of the board so that camping in there will be discouraged.