Twitch is really good in aram since you're always teamfighting which he loves a lot so yeah thats always fun to play.
In terms of your question, twitch definitely struggles vs Assassins due to the fact that he is an immobile adc which is generally the greatest target for those assassins, at this point it kind of depends which style you're playing, if you're playing AP twitch then you should have a slightly easier time since you have access to zhonya's hourglass, but still that isn't always enough, my biggest recommendation is taking exhaust if you know they're going to be an issue, zhonyas + exhaust or galeforce + exhaust is usually a huge amount of safety for you if you position well, past that if you still have issues another good idea is when you see your team grouping, try to stay out of enemies vision for as long as possible so they engage without seeing you, after you identify the threats to you and see them use abilities on your teammates it is much easier to approach the fight with stealth and clean up. I hope that helps a bit/answers your question. If you still struggle feel free to drop by my twitch stream, I stream every day so you can watch how I play vs those champions and ask any questions you have there for an instant answer, the link to my stream is in this guide :)