There are Spoilers ahead!!!
Can do this on Easy mode, then switch to Hard mode and do training camp (how I earned the achievement). I advise waiting to do this achievement until Chapter 6. You get Eyvind near the end of Chapter 4, who is already Rank 5. In Chapter 6, there's a village you come across called Reynivik, which has an event as soon as you get there. Make sure to choose the option to Rush to Help, as this will recruit Sigbjorn, who is already rank 5. As you're playing through the game, find four characters that you like that will live through the end of the game. Keep in mind that you want characters that you get early on to build them up, as you need 14 kills and 50 renown for each character to make them rank 5. You can use any character that lives until the end; I personally used Rook, Iver (he leaves in Chapter 4 for just a little bit but he comes back so don't panic), Gunnulf (can die in Chapter 3 trying to save a cart, just pick options ordering him to let go of the cart), and Tryggvi. You get all these characters early in the game, making them easy to level up, and they're the easiest to keep alive in my opinion. Once you recruit Sigbjorn, I suggest doing this right away because once you get to the next city (Boersgard), training camp is no longer available. If you don't like the set-up (enemy positioning or player positioning) you get when you do training, just quit out and go back and it will give you a different set-up. Can make it easier or more difficult.
For the battle itself, it isn't too tough, but it can be if you're not used to playing on Hard. The enemies have more armor and strength and they are much more numerous. Focus on the bigger enemies first, they can one hit your characters. Abuse the willpower system to get more damage out of your attacks. Instead of focusing on outright killing the enemies, lower their strength as low as possible. Reason being is when their strength is super low, most of their attacks will deflect off your armor, and if they do get through it does negligible damage. Also, more enemies in the battle means it takes longer for them to cycle through all their turns.
You can also use synergies with characters special abilities (use Tryggvi's Impale attack, then use Iver's Battering Ram attack). When using Eyvind, using his Arc Lightning, focus on diagonal positioning of enemies as it does more damage each enemy it passes through, but be careful you don't shock your own teammates! Gunnulf and Sigbjorn can both be incredibly powerful using their Tempest attacks as well as they generally have a high strength number, causing more damage to more enemies with one attack. Using Rook's special when your teammates are surrounding an enemy will cause him to do armor damage, followed by your teammates attacking them if they're in range.
If anyone else has any advice please let me know and I will add it to the solution, otherwise good luck, and if you don't win the first time just keep trying!