Hariyama
Use a Heart Scale to teach it Vital Throw and optionally, Arm Thrust. In fact, you could even try running Substitute (Move Tutor, Lilycove City Dept Store, on the terrace) + Focus Punch (TM01 found on Route 115) which can blow some serious holes through the E4 and Steven.
Aggron is sorely disappointing, and has no good STAB (Iron Tail's unreliable accuracy will get you stuck in a tough spot) and it is weak to Wallace. As much as I wanted to make it work for my own run, it requires you to specifically want to use it to make it work at par with the myriad other options available.
As far as your Clamperl's Toxic Stall strategy is concerned, be warned that E4 members carry Full Restores, so you'll quickly run out of Toxic PP by the second or third member. I know because I'm currently the same issue with using Toxic Swalot.
Honestly, I'd keep Sableye but train it at Mt. Pyre till it learns Shadow Ball, which will make it reliable against Phoebe. I also think you might struggle against Drake because your own Flygon is weak to Dragon types. I suggest getting TM 13 Ice Beam from the Abandoned Ship between Dewford and Slateport, and teaching it to your Tentacool/Tentacruel, or, with an X Accuracy, Blizzard from the Lilycove Dept Store.
Other suggestions include teaching Calm Mind to your Grumpig and watching it rampage with Psychic if you manage to get to +2. You can even teach it Shock Wave to help out with Glacia and Wallace, and I suspect you'll have a bit of trouble against the Champion due to a lack of brute force/ firepower, so be sure to have some setup moves and X items handy. Good luck!
P.S. Hariyama & Flygon should be more than enough to deal with Steven provided you can take care of Metagross and Skarmory with powerful Special type STAB moves.
