FL4K is unlocked with the Force Multiplayer achievement that requires you to win a match of Guardian on all of the Forces of Nature maps.
Ivory weapon skin is unlocked with the "I've Got This" achievement which requires you to kill 10 opponents as the leader, in a round of Guardian on a Forces of Nature map.
Forces of Nature is definitely one of the best DLC packs. If you're still playing and haven't gotten the season pass, it'll set you back 800 Microsoft Points. For the full weapon skin list, check out the official Epic Games site.