What do you do when a Diana on the other team has 31 kils and your team doesn't know when to engage?

I just had a game where I was playing Kennen and I was placed in a match where my jungler was Gold 5, I was Bronze 1, the mid laner wasn't even level 30, and the other two people were Silver and Bronze. Apparently the mid laner against the "newb" was Platinum and she's a hardcore Diana main with almost 1 million mastery points.

Don't judge me, I'm perfectly capable of climbing. I almost reached Gold last season and the only reason why I'm Bronze is I won the first 4 games in my placement matches and I just decided to spam 6 losses because I didn't want to wait to see what I was put in. I know when to leave lane, cs, take objectives, and all that stuff.

But this Diana rushed Luden's and every time she engaged this Syndra, she one shotted her, and about 25 minutes in the game she was already at her full build with full stacks on Mejai's from constantly getting fed off other champions. I was the only person she couldn't kill until the very end when she dove into the fountain to execute herself while the Nexus was getting destroyed.

Every time she Assassinated someone or missed a spell on a minion, it would proc Luden's Echo. I kept telling me team "ENGAGE HER WHEN SHE DOES THAT!" But no, every time she double ults, they're like:

"oh god, I better not touch that,"

and I'm like...

"she just used her entire combo, KILL HER!"

I engaged her as soon as this happened. And killed her, BOTH times!

How am I suppose to know when to assist my team when I don't even have my second item? The mid lane is getting wrecked and if I leave my lane, the tower will get destroyed and two lanes will be pushed in instead of just one.

Do I help my team when I know what to do and not risk the enemy getting fed, but risk my enemy laner getting ahead? Or do I stay at my lane and constantly last hit minions so I can help later, risking the outcome of the game by letting the rest of my team take over for the early portion of the game?