How to get this Achievement using a relatively doable strategy.
The fundament of this strategy is Alette lvl. 10 with Overwatch + Stravhs' Bond. All approaching enemies will be knocked back this way, and their turn comes to an end. I believe this is a well known strategy, but some people might not know it. It is very overpowered though, but needed for this.
The difficult part here is to get the item in the first place, because item drops are randomized. The item is lvl. 6, so I recommend getting Alette up to that level quickly, and then not use any items on viking funerals. The reason is that items don't drop as duplicates, so the more items you have in your inventory, the more it is likely that the next one will be Stravhs' Bond.
As soon as you have it, you can use the most useless items on funerals as you see fit. Until then, you should also only recruit "cannon fodder" units, that you don't want to use later.
The most difficult obstacles for me were battle #2 and #7.
#2: Make sure to have enough "cannon fodder" units in your party, that can shield Alette from damage. Use her Puncture to kill most units. Varl are a good choice here as well. Remember only Alette needs to survive, your cannon fodder can just be funeralized for Renown.
#7: Remove Alette from you roster (she will be killed probably otherwise) and put units in there, that can deal damage to multiple targets. The map is full with archers and obstacles, so it is important to clear a path to them quickly. Bolverk and Warhawks are good choices here.
General recommendations for your "endgame":
- I recommend a shield wall unit like Egil first in your starting order. Give him a +Aggro item and send him first using shield wall. Most enemies will focus on him. Give him Guts later on, to trigger Overwatch if a good opportunity comes up.
- Second in line should be Alette with Overwatch.
- Third should be a Skald that will stay next to Alette for his passive Skald's Song, so that Alette doesn't run out of Willpower. She will do all the kils, so every kill will grant everyone Willpower. Also use Tale Worth Telling on Alette, so that her damage is increased against stronger units.
- Rest of the line should be Archers with high Break like Yrsa, Canary or Rook, and Eyvind. The point is to break armor of incoming enemies from a safe distance and weaken them for Alette when they want to approach. It makes sense to give those units Ranged Distance+ or BRK+ items. Use Eyvind for mending Armor. You can also keep your Cannon Fodder units here until they die, before replacing them with the before mentioned units.