Invasions are crazy fun with black knight greatsword. Some tips:
After using a strong attack, the sword ends up behind you and out of sight for the enemy. The second strong attack happens to start from behind your body, precisely where it ended up after the first R2. This means that after the first strong attack, many players will see the opportunity and move in to attack you while you are "vulnerable", when in reality you are already charging a second strong attack.
The same goes for rolling strong attacks. It is quite concealed and hard to spot, especially when there are other things to take the host's attention away. Such as mobs or the host's own team mates.
The occasional experienced host can consistently dodge R2s, counter this by holding R2 slightly longer to roll catch.
Pretend you are running away when you are on low health, and suddenly turn around with an R1 or rolling R2.
Drink estus a safe distance away to lure them in to attack, then punish.