Event CE servant recommendations

Servants with good Buster-type NP. And off course 2-3 Buster cards
It worked more better for such Buster servants with ability of high star-abortion (no matter skills, or rider-archer classes) and good NP gain, like Gil, Ozy/Drake.

Not quite recommend for other lancers and sabers, except they have ability for NP charge and Buster-up (as you mentioned Mordred and Karna).
Berskerkers? if you have Waver for nuking, then also good. If not, then not ideal as limited/zero over

Wizard & Priest is a good SR CE in specific team and servants, like Vald of art team, or Archuria/Martha Ruler solo, Medea etc. Not quite versatile.

i think Wizard & Priest is good on Nitocris cause she has healing skill and can charge her own NP

Wizard and Priest is also good on Nero (heals+NP who does damage).

Yet, I have MLB Maid in Halloween, not gonna trade for a much smaller heal. Also, for NP dmg, I have MLB Kaleido Ruby (who gives uo to Arts as well, multiplicatively), Angel Song and MLB Dragonkind...

I would still try to get, if it wasn't so costly in the shop. Better to farm Hearts and Prisms.

The 5* one is good, but I got no gold CEs, buying 4 and leaving the feathers out is not an option for me...

It would be tough to use Heroine Elly on most berserkers, without having some nice NP battery supports, since berserkers tend to have such a tough time charging their own NP gauges. But there are certainly some situations where you could pull it off.

Personally, if I manage to MLB it, I will probably stick it on Kiyohime (Lancer). She dishes out some mean buster damage, and her NP gain is actually pretty decent, so she can use her NP fairly often as well. And since she is a lancer, her star weight is nothing to write home about, so CEs that increase crit damage (such as Joint Recital or Victor of the Moon) would be kind of wasted on her. So I think that Heroine Elly is a pretty good fit for her. Probably this could apply to almost any buster oriented sabers or lancers that don't have crit absorption skills. Elly herself would be a good candidate as well, since she's NP5 even.

Otherwise, if there is a servant with a mismatched NP type (they have a lot of buster cards, yet their NP is quick or arts) then maybe it would be good for them.

Wizard and Priest is kind of poopy.

You get the it, put that on any Buster servant with good NP Gain (Gilghamesh, Cleo, Carmilla, Ozy, Mordred, etc).

Those servants have a better option though, the mighty Black Grail. Anyone capable of NP spamming has Black Grail as their best CE really.

Heroine Eli CE is good for Buster servant that have battery skill or decent NP gain.
For now, i still search for the 5th Copy of Heroine Eli CE.
For faster nuke or NP on demand Halloween Princess and Golden Sumo is a good choice.
Also that Atk Boost from Golden Sumo is considered the best in general.
MLB Wizard and Priest more suited for specific team/servant, similiar to Maid in Halloween good for servant with self healing skill and good NP gain ( for example : Staller, Tanks, etc ).
Also specific doesn't mean it's bad or useless, it's up to your roster, servant Arsenal, enemy type and gimmick. It's stiff LOL, like mentioned above it is not versatile.

It's welfare CE's anyways and if you don't have the servant which can use those Wizard and Priest CE's, don't bother MLB it.

Bedivere is good candidate, single buster np with tactics and np charger skill, artoria lancer if you want to put it on lancer since she have protection from world's end and good np refund if you can follow up with quick art brave chain after her using her np, as for berserker use it on ibaraki because she have good hit on her arts and quick card so if you manage to get her abq brave chain it'll fill nice chunk of your np bar
Other option would be brave liz herself or nobu or any walfare servant that have np5 buster np