You seem to have forgotten that we were at war with Osama Bin Laden and his terrorist organization. Congress gave legal approval to do so.
Assassination outside of war is illegal.
I suppose that if ya’ want to do it present day German style, dotting all the i’s and crossing all the t’s (would Biden get that backwards?) you’d have Congress authorize actions against all the Iranian proxies who’ve struck us. That would pass easily - probably super-majorities.
Then that leaves the somewhat murky question of who actually directs the proxies, makes them effective with arms and training, etc. Definitely figures like Solemani can be taken out as military actions. Deep enough intel would likely reveal even higher.
Yes, Russia could argue something similar, but they know darn well what would happen if they hit any US / NATO leaders.
I don’t think it’s “legal” to declare war on an organization, either. That’s why we invaded Afghanistan instead of invading al-Qaida. Get real.