Unlike the other solution for this achievement there is an easy way to get this in one playthrough no matter what difficulty you play on.
The key to getting this achievement is keeping a high multiplier while in combat while using the "Bonus XP" perk (right on D-Pad). Each hit will add x1 to your multiplier increasing the amount of Valor you get when the combo is either broken or you've defeated all enemies in an area and the multiplier expires. An easy way to get your multiplier up is during executions. A Legendary hit will add x3 to the multiplier, a Centurion hit will add x2 whereas a Soldier hit will add x1 like a normal hit would.
Having your multiplier broken by an enemy will drastically reduce the amount of Valor you earn from that multiplier, whereas if you kill all the enemies within an area and the multiplier expires you will earn much more Valor. In the open square area while protecting your father Leontius in the "S.P.Q.R" mission I achieved over a x150 multiplier and earned around 75k Valor for that one multiplier itself.
This achievement is a lot easier as well if you have good combat awareness too, when an enemy is approaching you mid-combat always press A to keep them from breaking the multiplier and attack them and switch back and forth between targets when multiple are attacking you.
This achievement is fairly easy if you're good at maintaining multipliers. The rough cost of all the upgrades and executions combined is around 2 million Valor and all the upgrades and executions are unlocked by level 7 which is unlocked after earning 600k Valor.
Hope this solution has helped. If you leave a negative vote please leave some constructive criticism in the comments section so I can work on improving it.