Animal kills DO NOT COUNT toward this achievement.thanks EPSILON 616
You can accomplish this using several general strategies or methods, but here's how I did it. You may want to surpass 30 kills and go for 100 Atgeir kills to meet the requirement for an Ubisoft Connect core challenge. If you use the Atgeir for most of the game, then you definitely will get this achievement naturally. But if you plan on switching back to your trusty favored weapon as soon as possible, the next bit might help.
During the introductory sequence, you should get your first Atgeir, called The Impaler, after your fight with Sinmara. So equip it and use it on any enemies you see. Once you see the Dawn of Ragnarok title screen and your free-roam kicks in, attack the nearest mill (red raid icon) without your crew (don't use the Raid feature). Also kill any roaming enemies on your way there. If you still need more kills, attack another mill without your crew. Crew just get in the way and steal your kills sometimes. You can call your crew after you clean the area out so you can collect the Silica.
all Atgeir you can find:The Impaler
from first Sinmara fightThorgrim's Dying Breath
from underwater chest in Gapihus in SvaladalFiery Warsong
from defeating all 3 Sinmara's Chosen (Rammr, Throttigr, Trawll) and then visiting a blacksmith to forge it ("Can you fix this blade?")Muspel Spark
from completing the story for Dawn of Ragnarok
There's also an atgeir river raid reward from Vagn in your settlement, outside of the DLC:Balhrafn Atgeir
bought for 800 foreign supplies from Vagn
And another atgeir reward from Everold in your settlement after collecting enough Dreams from the Forgotten Saga DLC:Spita
bought for 50 dreams from Everold