Before we get into the invasions itself, you need to prepare.
First, start by summoning atleast 1 Phantom.
1 Phantom is for the Pro's though, so you are better off to start with 3 Phantoms.

As soon as you have these Phantoms, equip your loadout.
This loadout should include atleast one of the following weapons:
> Dark Sword
> Carthus Curved Sword
> Irythill Straight Sword
> ANY Katana
> The ''Legion Etiquette'' Gesture
> The ''Point Down'' Gesture
> A bad connection

As for Armor, you are best to use FULL Fallen Knight Set, best paired with any Katana.
This way, the Invader will cringe so hard, that he either immediately Black Crystals out of your world, commits suicide, or will be so shocked by you're cool appearence, that he disconnects.
A true sign of your astounding coolness.

Now we get to the fun part:
As soon as you see the invader, you call over all of your Phantoms.
Run straight towards the Invader, and spam all the R1 R1 R1 you got.

Don't worry, your gank friends are always behind you, you can't lose.

Dark Souls 3 made the Invasion System really amazing.
Myazaki and friends developed the ultimate invasion system, in which you, as the Host can almost never die. This includes:
> You can summon Phantoms infinitely
> You have 30% more Health because of Ember
> You can use Seed of a Giant Tree so even NPCs Attack Invaders
> The Invader only has half of his Estus
> You have atleast 3 Buddys watching over your back

The best way to fight an Invader is, to spam R1 all the time.
Remember: R2 and L2 doesn't exist. Its only R1.
But be careful! Don't go too near to the Invader, he could kill you. Better stay behind your Phantoms and ONLY hit the Invader when he gets stunlocked by 3 hits from your Phantoms!

If your Phantoms die, IMMEDIATELY sprint back to the next Bonfire, and summon them again. Should the invader try to attack you, just roll spam the entire time and wait for your beloved Phantoms to help you.
The picture Shown above is THE best strategy to really kill an Invader for good. You might even gank him so skillfull, that he leaves the game entirely!
And don't forget to use the ''Clap'' Gesture, or ''Point Down'' Gesture to show your superiority once and for all!

Should you be near death, without Phantoms theres also an easy way to counter that:
Just pull the plug on your router, or cut through your Lan Cable. This way, the Invader can't hurt you anymore! Isn't that a nice feature?

As a Blade of Darkmoon, its your mission to root out the Invader.
You can Gank with all the Phantoms together, or just sit around and do absolutely nothing! Both works! All you want is your Proof, and nothing more.

If you remember all of these steps, you will never lose to an Invader ever again!
The same tactic can be used on Bosses.
Why should you fight them alone when you can summon as much Phantoms as you want?
Its perfect!

I hope you guys enjoyed, and will soon use these tactics, in the beautiful World of Dark Souls PvP!