I've lost track how many times I've played "sewers" trying to get hardscale. 

I've used hundreds of mystery shards.

I need the hardscale to fuse for a champion - it's the only one I missing.

Am I being stupid and doing something wrong - or just having rotten luck?

Everything I read says play sewers on normal, or use a lot of mystery shards and you'll get it sooner or later - but I can't seem to get it?