if i play boromic as one of my heroes, can he use his readyskill to kill an enemy/ready/kill another and so on, or do i have to engage all enemies at once with my heroes/allies?

Yes, you can. The enemies engage a player, but not a particular character. You assign the order of attacks to be resolved one by one, so you can assign Boromir to attack Enemy 1, then ready him and assign him to attack Enenmy 2, and so on.

he can do this with all currently engaged enemies, that were engaged in the encounter phase, or left from the lest round, however he cannot defend, attack, kill, then draw another enemy from the staging area to defend attack kill and so on