So for these, a certain amount of strats are going to depend on the scaling. 200% bosses are very different from 500% clamped. For now, I'll call out where scaling falls off. I'm also including 3 of the unique early fights because without proper tools, they can suck as well.
MDEF notes: to get percent to hit, you take 148 + the spell's accuracy, subtract the enemy's MDEF, and then (this is where it's "roughly") divide by 2.
BANE and QAKE have 40 accuracy, ZAP! has 32, and CONF, HOLD and BRAK have 64. If an enemy is weak to an element, add 40 before dividing. If an enemy resists, remove that 148. 3 out of 256 times, things work no matter what.
Quad-X works 100% of the time at scaling under 280% scaling. His 64 MDef means that BANE and QAKE work a little over 60% of the time, BRAK works 75% of the time, and ZAP! works a little under 60%. Of course, his low overall stats mean that at regular scaling, any tier-2 damage spell plus any weapon on a fighter is enough here.
Literally any AoE spell (other than the HARM family) will tear through them. ZAP! is, like, 80ish% to land, and QAKE strats are in the 85% range.
XXXX is 100% to land at 170% scaling or less. Instakills aren't great here - BRAK is about a 20%, ZAP! is a 5%, QAKE is 10%. But unless there's a rude script, you can usually out damage him - the base attack stat isn't super high, especially with only 1 vanilla hit.
Obviously, for the most part? #DisrespectLich. In a lot of runs, you're going into Earth way more powerful than you have any right to. For your mages, Fire/Harm spells will do work, Lit in a pinch. He resists PSN/STN and Death so BANE, BRAK, and XXXX are right out... but notice the lack of QAKE resistance, which is about a 34% to land (or 1 in 3).
Kary resists your basic Black Magic damage spells; you're better off using that Black Mage to NUKE or buff the melee... or exploit that weakness to Status magic. Wanna put that Wizard Staff to use? Or paralyze something with HOLD? Kary's your window - both CONF and HOLD have just over 1 in 3 odds of landing, and both eat her turns - meaning that you can lock down the Fiend of Fire while you set up the buffs on the rest of your party. Otherwise, XXXX is always a useful finisher - just keep track of how muck damage output you've done.
OH GOD IT'S KRAKEN D: - as we all know, Kraken hugs *HURT*, so if you aren't cheesing this, you want to get up your FOG/INVS/Defense Sword/White Shirt casts ASAP. additionally, Lit > Ice > Fire for damage... but who has time for that? If you have BRAK, you've got a 26% chance to just end the fight - so that often becomes the de-facto Plan A for Kraken. Otherwise, just keep your melee attacker alive, throw out every NUKE and FADE you have, and pray.
Notice Tia's 200 MDEF. Notice that 200 - 40 = 160, which is the same MDEF that Kraken 1 had. That's right - both of these fiends are equally likely to fall to BRAK.
You might be thinking "Oh hey, WarMECH doesn't resist Time! ZAP! STRATS GO!!!"... except WarMECH's MDEF means that ZAP! is a 3/256. Treat WarMECH like an end game boss. Buff your melee, throw NUKEs and FADEs, and hope to not see back-to-back NUKEs and NUCLEARs in response.
ToFR is long. You have to face all 5 of the next fights on one set of charges; not to mention a lot of nasty random encounters. The best thing to do here is have a plan and make sure you ration your charges and healing potions appropriately. This isn't a guide on that, though, but it's important to not.
The final four fights kinda have the same flow - buff up, and hope for noodly melee attacks, weak scripts or paper defenses. Any one of them can be a huge threat or wall, though the deeper you get, the stronger the base stats. There aren't really any reliable ways to cheese, other than landing a XXXX on Kary 2, Kraken 2 or Tiamat2. Cheeky BRAKS are a 6% on Kraken 2 or Tiamat 2 - but that's not 0%. All other insta-kill is 3/256.
Everything dies to a FAST'd L42 BB, or a FAST'd Fighter/Ninja with a bunch of TMPR/Power Glove usages and a good sword. Everything dies to NUKEs and FADEs eventually. Be careful about the scripts - there's 4 NUCLEARs, 3 NUKEs, a CUR4, and Chaos' INFERNO, SWIRL and TORNADO in the pool. On the Insta-kill side, the pool has 2 BANEs, 2 RUBs, 2 XXXX's, and 2 ZAP!s - so mind your ribbons and WALLs as you go through the Temple.
Finally, keep LOCK/LOK2 in your back pocket for evasive fiends - landing 3 out of 10 hits means that fights will go a lot longer; upping the odds of landing stuff will save your bacon.
Other fun note: the Resistances on Fiend 2s are the same as their Fiend 1 versions - they just no longer have their weaknesses!
Here's the last really fun boss - you can still go for the QAKE strats - it's about 1-in-4. But given that by the time you're in ToFR, Lich 2 usually dies to the melee characters breathing, it's not really worth the charge unless there's nothing better at that level.
In really obscure strats, you can try to land XFER on Chaos (27.5% to hit) to turn BRAKing him from a 3/256 to a 6%... but that's a gamble that needs two specific spells and a lot of luck. (XFER has a 107 accuracy and, if it lands, removes the target's magic resistances)
It is, of course, possible.