First things first: for a more in depth conversation on strategies and such on the alternate final bosses, please see this YouTube video. That will give you some more in depth ideas for strategies on how to deal with these. I will put some quick tips, but nothing too in-depth, and they will only matter if you actually know which formation you'll be fighting.
Please note: if you have alternate fiends on, the alt fiends will replace the fiends you find in these formations. Same with WarMECH if you're using enemizer.
¶ WarMECH and Friends
This formation consists of two WarMECHs and Six Evilmen.
Quick Tip: Try to find WarMECH in the wild, and feel out how it fights. If it's got first turn NUCLEAR or something that does mass AoE damage, pack on those levels.
¶ The Fundead
Two Liches and four pumped up phantoms.
Quick tip: The Phantoms hurt. A lot. Try to drop them as quickly as you can. If you're fighting Lich2 and he's a bit more resilient than normal, that may also indicate that you've got a Fundead pack as your final boss. However, the Phantoms will tear you apart if you don't take them down ASAP.
¶ Double Dragons
Kary2 and Tiamat2 at the same time.
Quick Tip: Focus fire on one of them. I generally try to drop Kary2 first, as she goes down easier, but if Tia2 has some horribly painful stuff, you may want to focus fire there first. Just pay attention during the fiend refights to get information on them, if you can.
¶ Whack-A-Garland
Eight Garlands and one Chaos.
Quick Tip: Clear out the Garlands as soon as you can, especially if he has any AoE damaging skill. Even if Garland has something as low tier as HEAT, coming at you eight times is going to hurt. Additionally, if you have any AoE spells you can cast, pay attention to which target shows as Chaos, so you can have your melee start working it down, whilst the rest of the team takes care of the chaff. If you know this is coming, you may want to let the Garland fight drag out one or two more turns to make sure Garland doesn't have some nasty surprise waiting for you.
¶ The Sahagin Wagon
Kraken1, Kraken2, some WzSahags and R.Sahags
Quick tip: As you've got a Kraken1 mixed in with this group, you may be able to BANE or BRAK it… if you can determine which one Kraken1 vs Kraken2. As always, try to remove the weaker enemies with AoE spells tout suite. WzSahags can be particularly nasty at times, so clear that field of the dross as soon as possible!
¶ The Snake Pit
Tiamat1, Tiamat2, and a variety of Asps and Cobras.
Quick tip: Quite similar to the Sahagin Wagon, BANE and BRAK can be quite useful, if you can determine which one is Tiamat1. Additionally, similar to Whack-A-Garland, pay attention when using AoE magic to determine which spots are the Tiamats; however, as the snakes are fairly low tier enemies, you may not need to pay attention, as most of the time, any tier 3 elemental spell, or NUKE/FADE will very likely remove them all in one fell swoop.