If you are having trouble with the Oblivion gates, this works for all of them but one.

Get your illusion skill up so that you can cast invisibility without draining too much magicka.

You can use invisibility to get all the way to the top of the gate and remove the stone without ever fighting. The only exception to this is the Bruma gate where you have to "show the guards how its done". For this you can still use invisibility to avoid fighting, but you'll have to wait for the guard captain to make it through and assist him somewhat along the way.