So I'm considering using Oath against Fiends for my paladin. I have however considered a bad situation easily exploited by the enemy.
The Oath says "Never suffer an evil outsider to live if it is in your power to destroy it. Banish fiends you cannot kill. Purge the evil from those possessed by fiends"
Now anchoring aura gives you an immediate ability to cast dimensional anchor at a fiend trying to teleport away. This costs one of your smites.
Now, it seems to me that the oath would force you to use the anchor every time, since you cant let a fiend escape is you are able to stop it. The problem with this i think, is any smart fiend will send a bunch of lower level minions after you asap, to run into your aura and teleport out, and force you to burn all of your smites in a single encounter on low level enemies (As low at CR5 and 6's can teleport at will). Yes, the aura forces a will save to be able to teleport, but it isnt that hard to make.
I know that would be a little bit mean by the DM, but I feel it would be how a BBEG would act with a paladin crashing down his door. Is there any way around this? Or am I misinterpreting how things work?
Unless you've established otherwise in game, I would imagine that the exact wording of the Paladin Order's behavior restricting oath would be one of their most closely kept secrets precisely to avoid such exploitations.
I'm fairly certain no oath ever contained the phrase "Thou shalt not use thy brain; instead thou must be a fool for whatever ploy or abuse thy enemy has planned."
I wouldn't limit it in that way.
"Never suffer an evil outsider to live if it is in your power to destroy it" - is enforced by you starting to bash their heads in, trying to kill them all if possible, as far as I see it.
So you are saying as long as I attack any fiend I see, if I let him teleport away that will be ok?
I think that makes sense. I'll have to see what my DM says.
Side note... any way to give paladins the ability to banish?
If your character is reasonably sure that a greater fiend is sending in lessers specifically to weaken you, you are well within your code to refrain from using all of the abilities you have available to you, in order to save them for the last fiend.
Since by doing so, such a fiend has proven himself cunning, resourceful, and aware of your abilities. That is not the sort of beast you should fight on an empty tank, nor is it the sort you should risk letting go free.
Attack the minions, slay them well. Take the spellbreaker tree so you can smack them for trying to teleport away. And then get to their boss and end his shennanigans once and for all.