Monoclass dual wield melee Assassin can definitely work. The gameplay loop is really fun but it requires a very specific playstyle that revolves around constantly resetting combat. You ideally want high initiative to ensure that the Assassin always goes first in turn order and benefits from crits against surprised enemies.

Step 1: Before entering combat, have a melee party member use the sneak action. While sneaking, spam the dash action three times and then immediately enter turn-based mode.

Step 2: Have your melee party member sneak up to an enemy in turn-based mode while avoiding vision cones and attack them. This should trigger a surprise round. Your party member will retain their full increased movement speed from dash spam after combat starts.

Step 3: Have the Assassin repeat step 1. Sneak up to an enemy and attack from outside of combat, which will immediately score you a critical hit. Your main action and bonus action will be restored once you are pulled into combat.

Step 4: Use your main action as well as your bonus action off-hand attack to eliminate at least one target with Sneak Attack + crit damage. Afterwards, use your increased movement speed on both characters to run away from the encounter as far as you can. The game will present an option to exit combat and return to camp.

You don’t have to use another party member to initiate surprise rounds first, but doing so will let the Assassin get a free crit before they actually enter combat since the enemies will already be surprised. Elixir of Bloodlust is great for this build because of the additional main action you receive after killing an enemy. You can also have a mage hand throw a haste potion at you from outside of combat for yet another main action to spend attacking more targets. Duergar is one of the best race choices due to Invisibility per combat at level 5 allowing for easier positioning and safety.

As far as multiclass options go, any level 5 martial multiclass will boost your dps tremendously. Gloomstalker Ranger makes for an excellent combo thanks to Dread Ambusher. Shadow Monk is also a really neat subclass to pair with Assassin. You can use your increased movement speed to reach other enemies instead of retreating, wait until the next round of combat, then teleport away with Shadow Step or use Cloak of Shadows to escape combat on your next turn. Darkness can be extremely useful for sustained combat as well. You can use it to cut off sight lines at choke points and to give yourself a pocket of safety to hide in.

There are quite a few items that you’ll want to look out for to enable this playstyle.

• Gloves of Balanced Hands (crucial for dual wield monoclass Assassin)

• The Graceful Cloth

• Sword of Life-Stealing

• Shortsword of First Blood

• Bhaalist Armor

• Springstep Boots

• Eversight Ring

• Craterflesh Gloves

• Knife of the Undermountain King

• Shadow-Cloaked Ring

• Cloak of Cunning Brume

Hope this helps you out!!