Im not really a lissandra main, but i have m6 on her
If your enemy is at like 35% health or lower in lane and youre at least level 4, flash W them and do your full kit on them while they are rooted, while getting as many AAs in as possible. They should probably die if theyre a squishy.
Bring teleport and ask your other lanes to place wards far behind the enemies so you can teleport to them and get kills/assists you hit 6. If your botlane is there, i can almost gauruntee you will get a kill if you use your ultimate, and possibly two. Save your E to escape in case youre in danger during the roam.
Lissandra can engage late-game teamfights with ease.
Alert your team that you're going in, and then E>Flash>W>Self R into the enemy team.
This will chunk the squishies down to about 40% and immobilize them for your team, and will take some health off any tanks nearby while also preventing them from attacking your team.