I'm in a shard where Oinor has some incredibly tough Knights in his stronghold. My heroes levels 25, 23, and 18, and none of them can kill more than one unit when assaulting it. What do I do? Thanks!

hmmm to give you a better answer i would need more information
-what kind of heroes do you have? with what kind of equipment?
-Has Oinor only his stronghold left or does he posses provinces?
-What kind of units can you recruit?
and most important
-What difficulty are you playing on?

but in general i would say:
explore your provinces and clear the locations there for experience so you can get you heroes to level 30 - while you do that you should get some decent items
a high level pure warrior or pure scout should be able to kill 5-6 knights or paladins without even getting a scratch

Mage probably has the best chance, use some Mass Debuffs like Slow,Terror and stuff.
I dont see how a scout could kill 6 Paladins.

1. Grind them up a few more levels (level 30, 30, 25 sounds good)
2. Attack with all 3 at the same turn, each one will be fighting a now weakened enemy.