The other thing that really makes a difference for Assassins is the build. They aren't bad built as straight rogues, but they also aren't great. They work a lot better, solo or not, when they have more than one attack per round. That allows them to really make the best of their auto-crits when they've managed to set that up. I usually start an assassin build with two levels of fighter for the weapons and armour, saves, action surge, and fighting style, then go three levels of rogue for assassin, and then back to fighter for 3 for thesubclass, ASI and second attack. That same pattern also works well with Ranger 5 if you go Gloomstalker, the Gloomstalker and Assassin abilities stack really well together.
That won't help the larger problems of a Leroy Jenkins party, sadly.
That won't help the larger problems of a Leroy Jenkins party, sadly.