Invokers are at their deadliest in the 20's if you're going for a PK per hour stat. Clearly they are not as durable, but that's what makes them such appealing targets, and it's much easier to kill people who are trying to kill you first!

If you use every available advantage, you can do some damage as a low level mage, though sometimes it takes a lot of courage to do so. What's more, I am convinced that transmuters, conjurers and necros all have the ability to slowly solo rank. Invoker would be the slowest to my knowledge but still doable.