These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Mongbats, like demons, are powerful and fight in large numbers. They thankfully don't have annoying magic spells like demons, but are still overwhelming without using magic.
The worst of the bunch are the guards. They're quick and powerful fighters that band together just like mongbats, except they use ranged weapons, allowing them to stay in place and pummel you before your party can even move. The ones in Lord Blackthorn's castle don't even bother fighting you: they'll just swoop your party away and place it in the torture chamber, where one of your companions is guaranteed to be permanently killed off. The game makes it so you specifically have to avoid these enemies, rather than face them head-on as you would others.
The Magic Axe in this installment is by far the most efficient weapon in the game: it has the same range as the Magic Bow, but deals more damage, doesn't require ammunition, can also be used for melee combat, and can be carried in one hand. There are only two weapons that deal more damage, of which the Halberd is heavy and two-handed, while the Mystic Sword only works in short range. Walkthroughs swear by this weapon.
Porting Disaster: While the previous two games got pretty good (if somewhat limited) NES adaptations, this game's NES incarnation is a barely-playable trainwreck. Even more shockingly it was developed by Origin themselves, and while it probably makes some sense given that they had never developed for a console before, you'd think they'd care about making a quality product.