A lot of things of "you cannot do" have already been proven to be false.

You can attack a country with nuclear weapons, provided you use only conventional ones, then threaten to escalate to nuclear if they do that. Nuclear weapons are the ultimate threat which would ensure mutual severe damage if not destruction if used, therefore nobody uses them first unless they're completely nuts, or they're cornered. Putin is a criminal but he's far from being crazy, and as for now is surely also far from being cornered. Surgical attacks in Russia with conventional weapons would undermine his powers and create enough public disapproval to facilitate a coup from within, but should be done with extreme care and up to a certain point in order not to trigger a nuclear response. Sadly, even using conventional weapons, the number of deaths would be huge; it is entirely possible that Putin would sacrifice millions of innocents sending them to the front line before giving up, also because when dictators give up they usually die shortly after.

Manpower is what Ukraine needs.

Maybe that's for the best.