Summary
Of all the enemies we create for our players to face inDungeons & Dragons, the scariest ones to design are usually the bosses. You have to make sure they’re powerful and deadly but, at the same time, beatable. And they can’t be too weak because that would destroy the tension and ruin the moment. There are a lot of stakes in their fight.
Luckily, there are few abilities, features, spells, and resistances that can help you level the fight, and not much is necessary when the party is still at a low level. Whether you’re borrowing something from a class, species, monster, or homebrewing, there are always ways to make a boss more challenging.

Don’t add all these abilities to the same boss - they’ll be too deadly. Consider which options fit the character better or which are better counters to your players and run with it.
10Evasion
Avoid Those Spells
If you want the boss to be particularly challenging for your spellcasters,Evasion is a good trick you can take from the roguesand monks' features. Essentially, failing at a Dexterity saving throw will have you only take half the damage you’re supposed to, and succeeding the saving throw will let you take no damage at all.
In other words, the spellcasters will have to avoid using spells that involve Dexterity saving throws or risk it, and many high-damage spells use this one. Just bear in mind that spellcasters aren’t very strong at low levels, so this can render them useless in the fight, depending on which spells they have prepared.

9Second Wind
You Can Heal The Monster Without Having To Fudge, You Know
Another trick you can pull from other classes - this time fighters- is Second Wind; it lets you heal yourself as a bonus action once per short rest. Since bosses will likely not rest, you can instead use this as a recharge ability, giving you access to it back if you roll a five or a six.
Depending on how much health your monster has, it’s also worth increasing or decreasing the efficiency of the feature through the bonus that comes along with the die roll. You can also use health potions, but Second Wind has the beauty of working as a bonus action.

8Multiattack
Just Hit Them Again
The best way to deal with a group is to attack them multiple times. That’s usually the use minions have during a boss fight, but you’re able to have your boss cause a lot of damage on their own by giving them the ability to attack twice per turn - or more, but three attacks or more against a low-level party can be overkill.
A fun homebrew that you can do here is to let one of these attacks be replaced by a cantrip, giving your boss both melee and ranged options, such as hitting someone with a sword and using Fire Bolt on another enemy.

7Counter-Attack
Use Those Reactions
To make your action economy work even better against a group, you can have an additional attack that triggers your reaction. You can do it similar to the fighter’s Riposte, where you get to counterattack when the enemy misses, or you can do a counterattack where someone hurts your monster - your call.
Regardless of the trigger, the idea here is to simply attack more with a melee boss since once all other melee characters gather around them, people will likely not move anymore, and your reactions won’t be used for opportunity attacks. Make sure there’s something you may do with your reaction, then.

6Resistance From Nonmagical Weapons
Let The Spellcasters Shine
As mentioned before, martial characters tend to shine more at low levels, while spellcasters take a while to become super powerful - mostly because they can’t cast spells super often, and they won’t have access to amazing spells right away. You can make a boss harder while also giving your spellcasters a fun time to be in the spotlight through resistance.
A common resistance to have is against bludgeoning, piercing, and slashing damage from nonmagical weapons, meaning that all weapon attacks will be cut to half (unless the weapon is magical) while all spell damage will continue to be in their normal efficiency. Depending on your party composition, this is a good trick to pull on the fight.

5Cunning Action
Perfect For Ranged Bosses
Let’s steal one more from the rogues. Cunning Action allows you to use the Disengage and Dash actions as a bonus action rather than a regular action, meaning you can move yourself a lot more without having to sacrifice your attacks in the process.
With this ability, they can disengage from melee characters who have reached them without triggering an opportunity attack (unless the sentinel feat is involved). Dashing as a bonus action also helps increase the distance, but that’s also useful for melee bosses who can dash towards an enemy who’s far away and still get to reach them and hit them.

4Constrict
Hurt And Restrain
Restraining an enemy is a wonderful tactic due to its effectiveness. The problem here is that restraining usually involves using a spell. Grappling - the closest thing anyone can do - usually involves forsaking one of your attacks, and it doesn’t give you the same advantages.
Constrict usually works as an attack that causes damage and forces the target into a Dexterity saving throw (you’re able to use the monster’s strength to calculate the DC), and if they fail, they’re grappled by the creature, meaning their movement is zero until they’re free, but they suffer the consequences of the restrained condition, which means you get advantage on your following attacks.

3Gazes
Many Variations Here
Many monsters have a gaze ability, such as Petrifying Gaze, Stunning Gaze, Frightening Gaze, and so on. The idea here is simple: you force everyone near you (30ft is a good range) into a saving throw. Whoever fails receives the condition and can try again at the end of their turns to get rid of it.
To avoid spamming it, you can use it as a recharge ability as well or give a 24-hour immunity to those who passed the saving throw. Applying conditions to many people at once is a powerful way to increase the challenge here.

Petrifying Gazes usually use Constitution on saving throws, while things that affect the target’s mind tend to force them into a Wisdom saving throw.
2Spells
From Single-Target To Debuffs Or Small Area Of Effects
Magic always makes things more interesting, especially in a low-level group where the characters won’t have Counterspell yet to disrupt you. you’re able to’t go with extremely high-level spells just yet, but there are still fun options to go here.
We mostly recommend spells that affect a single target, spells that use your reaction,buffs to yourself or debuffs to the players, and some areas of effects that aren’t too big or too painful (a Fireball could be too much here).

Suggested Spells
Protection
Mage Armor, Shield, Shield of Faith, Sanctuary, Alarm, Cure Wounds, Healing Word
Command, Sleep, Entangle, Faerie Fire, Hex, Hideous Laughter, Hold Person
Damage
Eldritch Blast, Fire Bolt, Magic Missile, Thunderwave, Burning Hands, Spiritual Weapon, Shatter, Flame Blade
1Feats
Monsters Can Have Them, Too
You’re also free to add feats to your monster. Usually, features that go to players, whether feats or the ones we mentioned, aren’t super effective for high-level creatures, but they work fine here at the low levels.
For instance, crossbow expert can let your monster do ranged attacks without having to worry about proximity, or your monster can get extra attacks with their reaction if you add sentinel (and minions to help the trigger condition).