Allure's arm time might seem long, but the Charm and magic resist shred put Evelynn at an exteme advantage so are worth the wait.
While stealthed, pay attention to when you are (near) being detected by enemy champions. This is identified by the glowing yellow and red eyes over nearby enemy champions.
If at low health, you can take advantage of Demon Shade's healing and Camouflage to return to the fight and surprise opponents.
Purchasing Vision Wards can help you detect Evelynn's location in order to prepare for her ambushes.
A large share of Evelynn's threat is in her charm, 'Allure.' Protect allies marked with 'Allure' or, if you are marked, make sure allies are between you and where Evelynn may attack from.
If you suspect Evelynn is about to ambush one of your teammates, let them know by pinging the minimap and typing in chat.