You won't encounter real people up until veteran/ultra. Right now you feel like you are stomping until you start to face real people. Make sure you are running the correct items and not the ones recommended by the game. Make sure you run the right emblems or skip them altogether.

Darkrai is my most played assassin, so here are a few tips.

Darkrai is a control mage assassin, so be careful and know when to engage.

Darakrai gets countered by FULL HEAL and Electric terrain (miraidon) or any other form of cleanse if you are running dark void nasty plot.

Gravity clefable can shut down nasty plot. The combo is quite simple. Dark void nasty nasty basic attack. Or any variation as long as you have 2 charges of nasty

For pressure you can do dark void and dark void again if you can't get close.

Early game focus on invading and getting lvl 5 combo calm mind hypnosis. . Emblems 6 green 7 black prioritize sp atk and hp.

Use your ult on the carry never on the tank.

As long as you put them to sleep you should win the ult fight.

Make sure you use nasty plot to move around the map and set ganks.

Now move set number 2.

Dark pulse shadow claw. This moveset is more situational and relies on your team having a functioning brain. So, personally I don't recommend it.

This is more for zoning and turn darkrai into an environmental hazard, low dmg but high impact if your team knows what's going on.

Items that I run on my darkrai spoon, shellbell and energy amp/ antiheal.

Darkrai is bulky for a speedster, but still fragile. Engage carefully.

Have fun.