After getting Belias, Mateus, and Adrammelech, which one should I go for next? I am level 49 and have not made it to salikawood yet. Been grinding and hunting marks, hehe.

You can try your hand at Zalera. It's in the Barheim Passage. You need to have the Barheim Key, but it's easy enough to get where you're at. Just go to the Estersand Teleport Crystal and talk to one particular woman, the wife of someone, to get started.

Or you could try to get Cuchaillain in the Underground Passage from Lowtown. He's a bit harder than Zalera, but in my opinion, more useful.

ok I may try that one, I have the key already.

do you know what zone Zalera is in? Is he past Bloodwings zone?

yes, after that zone, there's a save point, so save and just keep going west and the zone after that should be the battle with Zalera.

Try Zalera.
But be prepared!
Your characters should be certain levels because of his LV. related attacks.
You should also have ALOT of white magic too!

Well I just finished Zeromus after an hour long battle. It was intense to say the least, hehe, but fun nonetheless. My next esper will be Zalera.

Try Zalera.
But be prepared!
Your characters should be certain levels because of his LV. related attacks.
You should also have ALOT of white magic too!

Bring plenty of Pheonix Downs and items for removing status effects like Chronos Tears; I'd also recommend setting up gambits for item usage to help make the party more effective.

As a side note, I was told that Zalera is immune to physical damage when his minions are present, however I wasn't able to inflict physical damage even with the minions defeated. It made the battle drag out, but I managed to take him down using 1,000 Needles gambits on my melee characters.

A good strategy I found for Zalera was actually quite simple.
Get Arise, Curaga and Curaja.
Have Arise be first Priority for everyone.
Then have second priority be Curaga on the nearest enemy(Zalera obviously).
Then when his other skeletons come up, Curaja on everyone.
Dont worry about healing your own people.
All he does is Death if you use this strategy(or so I found)

do you know where the map urn is (or if there is even one) for all the extra areas in the barheim passage

Thats a good question... Does any one know if there is one?