I have the Last Tantrum so I know what I'm talking about. In order to obtain the Last Tantrum you need to kill the Moon Lord with the Pink Guy's Set in your armor slot, not your vanity slot. You can get the Pink Guy's Set by opening a treasure bag from any boss, it has a 1/1,000 drop change from any treasure bag. Then after doing all that you will get the National Ugandan Treasure which summons Uganda knuckles. You cannot have defense above 250, you cannot have more that 1,300 health, and cannot have 50% or more damage resistance. If you manage to kill it with all those requirements you will get the Last Tantrum.