What kind of tough?
If you're at the bottom of the social climb in Frick looking up, to the left at the corner of the hill there is an uphill. It's cleanable if it's dry out.
The hill underneath the Forbes avenue overpass in Frick next to the dog park is always dry enough to complete.
Those hills are steep but they don't settle into your legs making them burn though.
I second the social climb to the left to the dog park, and from panther hollow lake to the intersection of schenley dr/blvd of the allies as better for that. Add the stone chute that goes up to the left in Schenley if you want to add a cherry to that sundae.
Also in Schenley, the hill between the golf course and cmu starting from the fountain to the stop sign is pretty good. Takes about 2 minutes up and 3ish back down if you make it a loop. It's not "tough", but has a wide shoulder; plus cars and the raccoons expect you there at all times.
Also not "tough" but steady, safe and in the city are Beechwood Blvd and Stanton Ave.