I'm doing my mono-rock runthrough of HG now, and it's going alright. I'm a little behind in leveling so I'm gonna have to do some GRINDAN ALL DAY ERR DAY, but other than that it's pretty easy. That's mostly because I got to choose my pokemon beforehand. I picked out 6 pokemon, produced eggs in SS, and transferred them over to the HG game and hatched them all. Then I kept one (Aron) and moved them all back to SS for storage. I get 1 pokemon from SS at each town I reach, which has been raised to an appropriate level equivalent to the wild pokemon in the area.
I thought it would be easy because I decided which pokemon I was getting and when, as well as giving myself the option of getting pokes that aren't normally available during the course of the game. This also let me choose typings that cover every one of Rock's weaknesses (Lileep for Grass/Water/Ground, Kabuto for Water/Steel, Aron for Grass/Steel, Lunatone and Aerodactyl for Fighting, etc). What I didn't take into account is that 3/6 of my pokemon don't evolve until level 40 (Kabuto, Lileep, and Anorith), and Aron doesn't evolve until 32. This balanced me out quite a bit and is making the run more challenging than I expected.
Morty is proving to be a little difficult, because the only pokemon I have that is able to sweep him completely through is Lunatone after a Rock Polish. Confusion can OHKO his Ghastly and both Haunters, but not Gengar, who can OHKO Lunatone with Shadow Ball. I have the potential to win if I can pull off a Hypnosis and get it to stick, but so far Morty's been employing some misshax. Nothing else on my team is high enough level to stand up to Shadow Ball, even Aron who carries a resistance. Hence, some Grindan is in order.