We are talking about wrenches...they aren't made of brass....

just teasing.
if you have rounded the corners and you can no longer put a 6 point socket or a pair of channel locks where you can grip it tight enough to get it moving.
Did you use some good penetrating oil on the sender threads/base of the sender.
At this point you are looking to replace it with new regardless. Time to break out the torch and heat up the base of the sender to where the heat can help break the bond. A set of locking pliers ( vise grips) can be used...try turning a little both in the counter clock wise and then a bit in the clockwise direction. Apply more penetrating oil and heat with more penetrating oil after heating the base of the sender very hot.
Another method (old school) that can get rusty fastener joints free is using Candle Wax or Gulf Canning Wax. Heat the stuck joint up to cherry red and then apply wax as it is cooling down. It pulls the wax in as it cools. It can take several applications but often is effective when dealing with steel parts...I don't know if aluminum oxidation has these same properties. The wax can burn sometimes and be careful of the hot wax...