Not all battlefield fights on Era are currently active. Following is a (partial) list of the BCNM, KSNM, and ENM fights available here. I will update the KS99 entries as I accumulate more seals to research, but contributors are welcome.
30 - Creeping Doom: An endurance brawl against a self-healing crawler. So the rumor goes, some jobs can even solo this fight... (Max 3 participants.)
50 - Treasure and Tribulations: Three chests, one of which grants an instant-win if you guess correctly and two of which transform into a difficult mimic fight if you guess wrong. This russian-roulette battlefield drops level 50 class-specific rings. (Max 3 participants.)
60 - Up in Arms: Try your luck at winning the jackpot prize of a Kraken's Club against this single sea monk. He attacks less frequently but for more damage as the fight progresses and his appendages are severed. Although this is one of the most straightforward BCNMs for new players, it is also done heavily on Era and the auction house is already filled with this fight's drops...save for the coveted and ultra-rare club. (Max 3 participants.)
Lachesis - Operation Desert Swarm: This excellent gil-dropping BCNM boasts treasure ranging from Warwolf Belts to Serket Rings, with six aoe-ing scorpions. Some truly skilled players have soloed this fight, others prefer a fast clear with a full group. (Max 6 participants.)
Atropos - Prehistoric Pigeons: Another excellent moneymaker BCNM, this a showdown against four Rocs that use bard special abilities and, as is frequent, grow stronger whenever one dies. Sleepable, bindable, manaburnable, tankable, this is a fight you could handle in a variety of ways. (Max 6 participants.)
lvl30 - Dropping Like Flies: Nine furious flies hover around a hairpin - find a way to handle this overwhelming number of foes at once and you'll get...well, an easily-farmed NM drop. Perhaps look elsewhere for a better use of seals. (Max 3 participants.)
lvl40 - Under Observation: Three cast-happy hecteyes guard a variety of money drops such as Utsusemi: Ni, Erase, and Phalanx, with Peacock Charm as the best drop available here. Kiting, pet-tanking, and shadow-tanking strategies are all valid. (Max 3 participants.)
lvl50 - Hostile Herbivores:Three uncharmable sheep guard a selection of level 50 race-specific belts and job-specific rings. Much like Under Observation, only you'll be put to sleep instead of paralyzed. (Max 6 participants.)
Clotho - Double Dragonian: A pair of shadow dragons - one a warrior, one a bard - will use two-hour abilities and become stronger as the fight goes on. (Max 6 participants.)
Atropos - Contaminated Colosseum: A single malboro with instant-death bad breath and a weakness to summoner zerg and blue mages. Cannonball your way to a cassie earring and more. (Max 6 participants.)
lvl60 - LegionXI Comitatensis: A showdown against four anticans on their home turf. Come prepared with aoe sleep and leave with a handful of the assorted level 65 torques. (Max 6 participants.)
lvl60 - Jungle Boogymen: The hardest part of this four-tonberry fight is the walk here. By now you should be used to the common strategy of sleeping and focusing damage on one at a time. Darksteel ingots are common here. (Max 6 participants.)
lvl20 - Wings of Fury: If you're looking for a more engaging place to farm for Astral Rings instead of trying to guess which chest is the lucky one through Treasure and Tribulations, these three bats with drain can be turned against one another by a skilled Beastmaster. (Max 3 participants.)
lvl30 - Petrifying Pair: The easiest BCNM for new players! No preparation needed, simply go slap some low-health lizards and maybe you'll get lucky with a pair of Leaping Boots. (Max 3 participants.)