I dont think one new skill will redeem all the mistakes made with this class... in fact, allow me to copy/paste my complete feedback on the class:
Compared to all calendar classes and even non-calendar classes, Archivist is the weakest and least-practical, not to mention very circunstancial class, with this AWFUL cooldown mechanics that, IMO, simply ruined what was going to be the cutest, nostalgic and ingenious class of all the calendar collection. It's simply ridiculous how the balancing screams "DO NOT USE THIS CLASS", how the decisions made it a waste of time (pun intended) and even the mechanic of some skills are unpractical, such as the stun. It's not a full chance, the chances are not each hit, but the total of the auto attack, and it's one turn, so you have to use it again and again to keep the enemy down, which ALSO means you have to first pick other skills to complement your attack with the stun being the last to, well, last longer. It's basicaly a setup hell. If I could "fix" this class in a rebalancing...
-CUT OFF the cooldown carry-over. There is no argument, not a single player agreed with this mechanic. Just remove it entirely, there is no one asking for it to be kept.
-Reduce Celestia's heal and also give it a proportionaly normal cooldown. Or make it a "reset heal" like Pyromancer's Phoenix reborn, that have a good excuse to carry over the cooldown.
-Give it a sub-par useable heal maybe a 10% HP HoT or a leech.*
-Give it a shield*
-Give it a blind (optional)*
-Give the stun a 100% chance like all the normal stuns in game but cut the empowerement from 5 to 3 turns to make it fair.
-Reduce the Ultimate Orb turns from 5 to 3 or 2 to make it fair.
the skills with * are to be suggested with these "memories"
Shield: Aegis casting a shield on us, Ash defending us from Caitiff's blast, or better yet, Dove doing the same. That "belly shield" would be awesome to use it whenever avaible xD
Blind: Sek Duat, quite easy. Using any of his usual skills in the bossfights.
Heal/Leech: Nythera using her void skills. Frydae draining Thursday. Anything that goes well with draining HP.