Hello everybody, im just adding this solution cause i have read in the comments above that multiple kills with a grenade tag does not count towards this achievement and im actually surprised that after all these years nobody wrote a solution that multiple kills with a single grenade tag actually does count. I have unlocked this achievement few days ago (March 2022) and i can confirm that multiple kills with grenade tag does count towards this achievement. We were playing Warzone, usual set up as you would go for a Seriously achievement except that we put 17 rounds (instead of 19) cause we were going mainly for weapon kills, We had 3 dummy accounts so we had to complete 34 rounds to get the 100 kills weapon achievement( you get 51 kills per 1 game = 17 rounds). So how this worked? We found a grenade on the map, you get 2 grenades per pick up, we were playing on Mansion ( but you can play on any map it does not matter) we ran up to dummy accounts, we put 2 dummy accounts next to each other and tagged them, you got a double kill and then grenade tagged the last dummy account it took me and my second teammate to get this achievement in 2 games (34 rounds) each……all of the 3 kills per round counted as a grenade tag, i cant confirm if this works for triple kills, we havent tried that to not mess up the counts, but for sure it works for double kills.