To me it sounds like the only reason to buffing most of these is because “more powerful mythics exist” which imo is inherently the wrong approach to increasing diversity. Very few of them are plainly bad. Also remember, that these are the most powerful monsters in the game which means even a small buff can have devastating effects. Usually when a mythic is restricted, it is that for a reason. However, I see some justified points being raised.
-Onyxia is complicated. It can do many things but isn’t great at anything. The nerf to the sleep “cycle” kind of screwed it a bit because before you could skip right behind a sleeping monster, give it a fatal sting, and it would die 71 seconds after it gets a turn. Dreaded wave has potential but kind of works against the control strategy because you hit all the monsters with it. ELEM Healing also doesn’t really fit the rest of the moveset or the profile of the monster so I’d like it to have LINK Cannibalize (3 uses) instead. Fatal sting could still have its uses against buffed enemies.
-Excalibur is a tricky one. With the 42 TU piercing move it’s one of those that will completely break the game if it’s buffed too much. Maybe an ever so slight increase to the crescendo powerup because it’s indeed very slow to get going.
-Auraleus. You can use the SS and give turn or charge it with ally monsters and create a loop. Requires setup but it’s one of those that needs to have that requirement. No changes needed imo.
-Persephia. Has a potential to snowball. Got a buff already.
-Lime. Meant to shell lock the opponent. Tricky to use but has its niche. Gaia awakening makes it easier for her to do her shenanigans but the 100% attack buff is imo completely wasted on her because she has only one attack of 170 TU. Needs another attacking move, e.g. Union Attack.
-Noircyon. The active moveset is weak with too many conditions. Also has to sacrifice a teammate for a spiritwyrm (at least the Spiritwyrm is very strong). I think this monster would greatly benefit of having a blood move, e.g. Bloodlust instead of LINK shield ally.
-Tyrfina. Benefited from the From the Brink buff. She does her thing and potentially sets herself up with the diamonoids. This I think is a clear case of “should be buffed because others are stronger or easier to use”. I would swap Bloodcrush for Confident Strike.
Other mythics that need a little something IMO:
-XYZ-999L. With so many stun counters and converters roaming around, the passive is just stupid. I’m usually against pseudo conditions but this time I think a 25 TU daunting field would be perfect.
-Rattaur. It just sucks. I guess Revenge Breaker is good but it needs that attack boost before being able to get kills. Shield ally on this monster is completely useless. Not sure either what’s the reason to have the “Reproduce” move. Not really sure what to do with this one, I think it’s just unnecessary.
-Medbie. Basically a mess. Does her gimmicks which don’t help to achieve anything really. Maybe I’m missing something and it can be used in comboes but this is another unnecessary mythic I don’t know what to do with.