A psych demon that send blasts that temporarily mess with controls and UI while sending waves of regular enemies at the player.

A bigass walking wall.
Like a really really long Whomp.

An imba cheatcode spells throwing Demon !

An anime Demon Lord Waifu.
How can the Lone Warlock beat that?

A sneaky tunneler demon. One that can tunnel underground and heal then pops back behind its prey for a bite.

Time ⁢

Can he handle Dante from Devil may Cry?.

A virus demon.

Guns and swords don't cure fatal diseases.

I would summon a demon as deceptive as it is deadly, and I would call it:

Venossis

This thin, bone-like serpent may not seem like much, but it can condense it's body to fit through any gap, thus being nigh unstoppable.
It can ignite itself on command, creating a vast, kamikaze explosion after a short time.
The Venossis also has a toxic bite, that can kill an elephant with a single bite.

Overall, this whiplike menace can put up a fight. It's only weakness is hot lead.

A demonic, twisted version of the Archangel Michael

For my demon , it would be like no other-Firstly , there's Normal demons , that can ONLY POSSESS the corrupt or those unbaptised . Then there's HIGH LEVEL demons that can possess NON corrupted or HOLY people-BUT there's a SPECIAL DEMON , MENTIONED in the book of genesis-For this demon can POSSESS not just deadly car or bikes , BUT ALSO inanimate objects and MORPH THEM , like TURNING ON THE BLADES of a pedestal fan when a person CLEANS IT -BUT they can also TEMPORARY MORPH that solid object into any NIGHTMARE SHAPE , TEMPORARILY. This SPECIAL DEMON is NOT QUITE classed as either a demon or man-This SPECIAL type of demon , being UNIQUE is referred to as the NEPHILIM in the BOOK of GENESIS . WHEN IT KILLS it's INFLUENCE GROWS STRONGER , INCREASINGLY WARPING in AREA/SIZE TIME AND SPACE itself . These WARPS of time + space STARTS OFF VERY SMALL , but as it GROWS IN POWER , then so does the size/AREA'S time + space that's WARPED-Eventually the landscape will morph/change into a MOCKERY of what's normal until the ENTIRE UNIVERSE will fall to it's INFLUENCE , WARPING the very fabric of ALL realities in EVERY PARALLEL universe. Even the gods themselves will WINK OUT OF EXISTENCE , in favour of this new WARPED REALITY !

Here is my idea: a demon with the ability to reflect the damage thrown at it back to the warlock.
The demon would have 2 modes of reflection. One for physical damage, in which it would reflect bullets, other projectiles and even melee attacks. The other it would reflect magic damage only. Both reflection modes should reflect damage right back at the warlock, with a homing element thrown in, with maybe a small speed reduction thrown in when possible, to balance things out.
Switching between modes should take a couple of seconds, with an obvious model animation, and there should be some obvious way to tell which of the modes is active, like the color of the demon's armor or an aura effect for each mode.
Alternatively, both effects could come from an item that could be dropped (possibly slightly damaged) by the demon upon death and equipable by the warlock.
Like a shield that is a demon in item form, that looks one way (ex. mirror round with demonic eyes all around its circumference) for reflecting magic, and another way (ex. tower shield with a big maw that eats projectiles and spits them back cloaked in a homing aura) for reflecting bullets and other projectiles and attempts to eat the warlock's arm when in melee range, producing a short immobilization effect that can be cleared out by some kind of melee magic attack by the warlock (ex. melee arm gets swallowed, warlock can cast shock or fire with his hands and the demonic shield lets go), but in the mean time, the other demon that is holding the demon shield can get an attack or two in.

Maybe the almighty and irresistable TAX COLLECTOR (rumbling thunder in the distance).

A demon that is a shape sifter that you digital information like phones, pc, and social media. Help it torture the victims. With there darks fears to keep it young its original age is when we first started using first computers and it’s original form is the punchcard for the first computer but looks like a teenage girl who can change her age as needed. And her is Codey.

A nightmare who hunt our fellow witcher in his dreams infecting his mind until he no longer sleep or rest and then any real creature defeat him