Hexblade is only functional with pact of the blade. So that's picked. Now, stats wise, lets pump and dump as much as possible: 8, 16, 16, 8, 10, 16 with a half elf. Medium armour and a shield, because that's that kind of silliness.
Skip greatsword, it's a waste of time. A longsword and 2 AC are much smarter, especially since you're working with a d8 hit die.
Lets start the cheese.
Darkness + Devils Sight: Perma advantage to hit and lol, you're invisible. Improved pact weapon will be your other invocation. Your 4 spells should be hex, darkness, hold person and armour of Agathys. Take elditch blast, toll the dead and one other cantrip.
That's basically it. Cast darkness, seek out the mean mofo, and cut him up.
Now, various human is a waste of time, and so is great weapon master. Why? Because despite this classes appearance, and be clear with this: You are in no way a frontline character.
Lets have a look at the three actual frontline characters: Fighters, Barbarians and Paladins. First, d10 hit die. Second, they all come with a solid endurance enhancer: Heavy armour / shields / resistances. They also come with heals, or an outright refusal to die. They all come with solid, reliable extra damage: rage, superiority dice (does anyone play a fighter that isn't a battlemaster) and smites.
If you really want to get cheesy, go for Paladin 2 / Warlock 3. Go for Str instead of Dex. Now, angle for plate + shield asap, and you're at ac 20, at level 5. You have 2d10 and 3d8 hit die, 10 points of lay on hands, and divine smite. Take Duelist fighting style, and the sentinel feat as v human. Also, two extra first level spell slots.
As a 3rd level warlock, yet, a 5th level character, you can pick up.... Thirsting Blade alongside Devil's Sight.
Start a fight by casting darkness, then wander in in 20 AC, with disadvantage to hit, with advantage on all attacks, smites (without invocation), heavy armour,+2 damage on hit, and reduce speed to 0 on AoO, which you get lots of cos who wants to fight an unhittable, unseeable opponent?
The more I think about this, the more thoroughly disgusting it seems.