If you aren't afraid to spend a little cash, look into the God Hand nippers. They are typically used for Gundam model kits, extremely thin and incredibly sharp. Using these will ruin all other plastic nippers for you, seriously. They are extremely delicate, but they cut through plastic like butter and leave almost no noticeable nub-mark behind: https://www.amazon.com/God-Hand-Ultimate-Plastic-GH-SPN-120/dp/B01MUGEO9X

After shattering my pair of God Hands on a piece of sprue that was a little too big (did I mention they are delicate?), I switched to these - similar to the God Hands, but a little less delicate and much cheaper for what I would call a similar result: https://www.amazon.com/Valtcan-Prime-Model-Nipper-Blade/dp/B01M5AOPN6