The inverted row isn't just for beginners. More advanced lifters can use it to build massive forearms and crushing grip strength. Just use a thick bar or Fat Gripz attached to a regular bar.
Set up a thick bar in a rack or use Fat Gripz.
Get a good grip on the bar, pull, and pause for 1-2 second count at the top.
After 7 or 8 reps, you'll really feel it. Ramp it up by wearing a weighted vest if needed.