I found a way that really helps you to get 3 gold medals in combat and achieve highest kill streak. I don't know if other people found out about it but you see there is that pistole logo next to your sword and those throwable knives next to your knife for a reason. when you are performing chain finishers after your first counter-attack, when you press X to kill another very quickly just simply Hold
and you see that Ezio stabs the first guy with his sword/knife and kills another guy with pistole/trowable knives . after killing both enemies, Hold
again to kill another two. the only difference is that you hold X and get a double kill.
the best thing about this double finisher is that when you enter this short animation, the other enemies won't attack you like they usually do when you are in the solo finisher animation. at the start of all 3 trainings that requires you to achieve highest kill streak, you have 15 trowable knives and 10 bullets. if you use all 15 trowable knives, you get a total of 30 kills! the score you get if you even reach half of this point is nearly enough for the highest kill streak point you need in the final training. I used my Knife for this. it's faster than the sword in case you wanted to perform a solo finisher in the between. also, I got this in less than 5 mins and I hope you guys can use my guide to get this achievement easier.