I made a simple guide about how I got this achievement.
A few important things to note,
Map is "
The Last Exam" found in Arena 6 (25 Medals to unlock)
1. Focus on main enemies and execute the others when you get a chance,
Lightning Strikes are useful (Light/heavy attack after a successful parry). Time pauses during executions and killing normal enemies give you some focus to help with main enemies.
2. Heavy attacks is all you need to kill 1st and 2nd main enemy, don't try using focus on them if you get any.
3. 3rd enemy is where trouble starts, you'll most likely have 1 focus bar for her so use it. Always do "
Face Smash" (requires 1 bar and does alot of damage) and follow up with 3 heavy hits. 3rd hit knocks enemy back and if you get lucky, the enemy will get knocked back into a wall and you can do another 3 hit to kill her easily.
4. Last 2 enemies are same story as the 3rd one, Face smash followed by heavy hits.
It took me about 30 attempts to get Gold on this level and now I can do it fairly consistently, Keep trying you'll get it eventually.
Here's my playthrough of it