Breather Boss: There are many of them, particularly if you use the right weapons. Some of the more notable examples are:
Bombardier. It teleports around the room and launches a ton of missiles each time it appears. Thing is, the missiles do very little damage and quite frequently drop life when destroyed. Bombardier itself is easy to dodge too, since it doesn't move save for its teleportation and it always gives plenty of time for you to move away before phasing in.
Special mention goes to Red Bombardier at the end of Corridor 21, he is fought at the end of the Boss Rush right after Grimgrin and right before It. He's basically just there for you to collect health drops from him and refill your life.
Teramute. Like Bombardier only even easier. Teramute teleports around and launches a spray of fireballs in an arc in front and to the sides of itself. Like Bombardier, the projectiles do little damage and the boss is exceptionally easy to dodge. Unlike Bombardier, who launches missiles all around itself, all of Teramute's fireballs come from its mouth, so you can just get right below its face and hold down the saber laser, instantly destroying all the fireballs and doing massive damage to Teramute in the process.
Glider, the boss from Corridor 16, if you know how to fight it. It's usually invulnerable and sweeps up and down the screen, but this attack is extremely easy to dodge. It then stops and opens itself up for attack in order to launch a few shots at you - hit him with the Saber Laser when he stops and he's done in about four hits.
Zibzub, the flying squid. His up-and-down movement is easy to predict and can be dodged by just moving back and forth slightly whenever he moves back toward the top of the screen, and the tiny squid projectiles he launches do very little damage.
The Eyeball Growths. They're both basically masses of eyeballs that function as stationary turrets similar to the first boss in the game, except by the time you reach them you have tons of weapons and health. Plus, there's tons of things in the battle for you to destroy for health pickups. Definitely a breather considering the region they appear in is also home to Grimgrin...
Cult Classic: Despite being unknown to a wide audience, the game is highly popular and appreciated in retro gaming circles.
In the last half of the Corridor levels, there is a blue ship enemy that loves to fire lasers. Each laser does more damage than any of the bosses can do to your character, even after having all the armour upgrades. The fact that this enemy comes is large numbers doesn't help, either.
Near the end of the Corridor levels you'll start to run across hopping blue mechs. Not only do these mechs have the same extremely powerful laser as the above-mentioned blue spaceships, they also have a much more difficult to predict movement pattern, and are immune to Enemy Erasers.
Many of the small mooks, especially the various multicoloured jumping jellies in the labyrinth stages, as well as countless types of enemies in the Corridor stages. The labyrinth areas have bats, but those are not the enemies you should be worrying about.
The green eyeball things in the Organic corridors. They don't attack, but they do absorb every single one of your weapons except the Enemy Eraser, and in order to beat them... you run into them to instakill them. Not an easy thing to do when there's other enemies around, and your shots get blocked by these guys...
The large blue crystals in the Corridor stages. These have a lot of health, attempt to slowly move into you to deal very heavy Collision Damage, and are immune to the Enemy Eraser.
Good Bad Bugs: If you have enough chips, you can buy all of the items in the shops where the Blue Landers are offering you multiple objects for sale by pausing very quickly as you make your first purchase.
Nightmare Fuel: A few of the bosses, but Grimgrin especially. Although his normal form — a freakish floating head with lots of eyes— is already creepy, it reaches a whole new level as your attacks blow out his eyes, leaving gaping sockets...and then a flood of tiny, damaging eyeballs pour out from them, filling the screen.
Scrappy Mechanic: The Password Save feature, despite (bizarrely) being the major selling point on the game box. The Guardian Legend was released towards the end of the era of password saves, when games were getting too complex for passwords to be efficient, and it shows. How bad were they? They were 32 characters long, including both upper and lower case, plus numbers, and the symbols "!" and "?". The awful passwords were considered to be the game's greatest weakness by far.
Eyegore uses a similar attack pattern to Grimgrin, but launching skulls instead of eyes, and combines that with a highly damaging rain of laser beams. Red Eyegor trades out the skulls for tiny (so they're harder to destroy) but highly damaging homing needle projectiles.
There's also Optomon, the boss that looks like a tangled lump of seaweed with an eye in the middle. They also fire long lines of vines that will inflict an unbelievable amount of damage to you in a short amount of time if you get caught by them. The green one is mildly challenging, but the blue one is very, very tough to beat. By the time you get to the red one, however, you're much stronger and better prepared to deal with it, so while still challenging, it's not as hard.
The Final Boss, It aka The Final Guardian, uses similar attack patterns to hated boss Grimgrin, all the while launching countless waves of small spaceships that home in on you and small balls that rain down on you from the top of the screen. Couple this with two Combat Tentacles with nasty claws that make it difficult to get in front of him to attack.
On the surface-traveling stages, the mini-bosses that look like jumping balls of seaweed with mouths, especially the blue and red ones. The way they bounces up and down makes it difficult to circle around them. And just when you think you've got its movement pattern down and can kite it successfully, the red version comes along and completely changes the pattern.
The "ice crystal" miniboss is another tough one. It follows you around the room... sort of. Sometimes it stops, or changes direction abrubtly, making it difficult to kite. Add to that the shots it fires which move in a strange arc which is difficult to predict.
There's also the one-time-only "dinosaur skull" miniboss, which flies rapidly and unpredictably back and forth and fires extremely damaging fireballs at you that travel like the ice crystal miniboss' shots.
Vindicated by History: Although it did get nominated for a few awards in Nintendo Power, most other professional reviews of the game at the time of its release were negative, with Electronic Gaming Monthly in particular calling it "only average at best", and the highest rating it ever got was almost an 8 out of 10. Fast forward to the 21st century and you find it on a lot of lists of best NES games, with IGN calling it "one of the most influential games in the history of the gaming industry" in 2009 and Gamasutra calling it "one of the best games ever released." Most of the credit for this rests with the game's complicated, 32-character long Password Saves which were a huge downside at the time, but not so much anymore since most NES gaming these days is done with computer-based emulator programs, where save states remove the need to worry about those obnoxious passwords.