It seems that every boss fight has a random chance of including some monsters from the floor, so it's like boss + random encounter. The interesting case is the one in which there are at least two rows of monsters and boss is placed in the third row. If boss is in the third row he cannot do anything until one of the other rows is destroyed.
1. Enter the fight and reload until boss is placed in the third row.
2. Disable monsters in the first/second row or leave one in each row and just ignore them.
3. Cast heals, all buffs and debuffs, lower pary AC etc.
4. Cast any spells you have to increase damage dealt by Zeo Nadar.
5. Kill monsters in the front rows and unleash the boss.
6. ONEShot boss with Zeo Nadar.

Why is it possible? In Wizardry or Bard's Tale the mosters groups can "advance" changing their position in fight, but it looks that they cannot in Elminage Gothic, so bosses are waiting while I'm lowering my party AC to -40.

Of course, this strategy might not be advisable if certain troublesome monsters appear.

For instance, if there's an Archer Polete involved, you probably won't be able to pull this off; the Archer Polete can attack twice per round, hits 4 times per attack, does over 300 damage per hit, and has a chance of beheading with each hit, and there's no convenient status vulnerability. From what I have read, there is an even stronger enemy called a Stargazer, but I stopped playing before encountering any.

Some enemies have a skill that allows them to automatically recover from all status ailments, including paralyzation and petrification, which makes it impossible to completely disable them.

Also, eventually you may find that -40 AC isn't good enough.

One more thing: Playing a song every round will increase the power of your songs until you spend a round without playing one. (It doesn't have to be the same song each round.)

Thanks to this trick I killed Cyclanos in second attempt (first time there was only one row of monsters) and my party wasn't really prepared - I wasn't expecting final story boss (didn't even explored volcano dungeon). This is very true, but so far every dungeon floor seems to have wide range of monsters - some very dangerous and some very easy, so one can reload if boss spawns with some tougher monsters. It may be different in the postgame, ofc.