You can get this seal to complete by finishing one of the gilded triumphs that still pops before you finish the seal. It is the one that has you kill 50 enemies with the same weapon archetype as the Seasonal ritual weapon archetype, which is a Rocket Launcher. Props to my clanmate, Quill, for finding this. Edit: You will not be able to gild the title until the Wyvern bug is fixed.