The phrase "final boss" refers to the ultimate and most challenging enemy or obstacle that a person, usually in video games, must defeat in order to complete the game or level. Full definition
When we have unlocked enough territories to threaten the bugs enough, they will launch a massive attack against our headquarters, headed by the leader or final boss of the island. (indiewatch.net)
It's up to you how much you fancy diving into the tale, but by doing so you're rewarded with extra hearts, more stamina and ultimately an easier final boss fight. (alphr.com)
Honestly I find most final bosses in video games easy, especially the ones with item management. (thevitalounge.net)