Thanks for your amazing efforts and looking forward for the release! My suggestion is that you don't make any different key combination. Simply, when any enemy is very close to Master Chief, you automatically activate melee instead of shooting and you show a different animation from shooting, while also inflicting a significant (fixed or proportional to their HP) amount of damage to enemies. Given the projectile limit, if you cannot show a melee animation, how about pushing the enemies a little bit back, while enabling their damage animation and sound, WITHOUT activating a fire animation? This way, you need no additional keys, no additional animation, and you imitate it by response of the enemies. If you can add a bump sound as well, you are set!