There are a total of 27 Aces/Captains you must kill (the rest are automatic campaign progression and are not possible to miss). To kill them you will have to find the yellow dots in the space battles which indicate that they are aces. If they are not aces then they will be on the ships you attack, these are again not possible to miss as they are part of the mission, however they are all on side missions which are missable from the main campaign. If you have missed some then go back and replay the mission on recruit and kill the aces and pilots before you kill anything else so you do not miss them.
Not all the aces or captains will be immediately visible in the fights as they may come later in the mission or when a warship is destroyed (you have to kill the warships to progress the mission) however I have listed them below incase you have missed a side mission. You are able to select the mission you want to play from the main menu and upon killing the aces do not quit out straight away as the progress will not count, you will need to wait to reach a checkpoint.
Operation Port Armor
-Cpt. Bradley Fillion
Operation Blood Storm
-Cpt. Rion Jones
-Sr. Lt. Khoa Le
-Jr. Lt. Moudy Hamo
Operation Phoenix
-Cpt. Tara S Emin
-Col. Young Kim
-Maj. Nicolas Rado
-Jr. Lt. Cesar Magana
Operation Deep Execute
-Vice Admiral Caleb Thies
-Cpt. Oren Emin
-Cpt. Khosi Siyada
-Jr. Lt. Andrew Hawryluk
-Sr. Lt. Omar Gatica
-Lt. Matthew Tovar
Operation Taken Dagger
-Cpt. Lt. Radoslav Barkov
Operation Trace Kill
-Sr. Lt. Suresh Malakar
-Cpt. Yenal Kal
-Sr. Lt. Taylor Kurosaki
-Jr. Lt. David Schorn
-Lt. Tom Szakolczay
Operation Grave Digger
-WCdr. Ivan Perez
Operation D-Con
-Vice Admiral Vlad Derhachov
-Lt. Rulon Raymond
-Lt. Serozh Sarkisyan
Operation Sudden Death
-Lt. Col. Damien Nichols
-Lt. Sergio Gil
Operation Safe Harbor
-Sr. Lt. Attila Zalanyi