If you see Evie uses Ice Block and stays in it to heal, Charge your shot and aim for the head. there is a small window where Evie can't take action after Ice Block and you can one shot her as she comes out.
When sniping an Evie who is about to leave Ice Block, aim a little bit in front of her path in the direction she's facing because she will usually blink after leaving the block and tends to walk forward a bit first.