What kills Rollout is that it stops as soon as it misses, which means you're not going to be doing a single HP of damage 55% of the time. And the power rise just doesn't seem that big from what I've seen.
As for your moveset, here are the ranged moves you can choose from: Water Pulse, Ice Beam, Shadow Ball, Shock Wave, Flamethrower (and Focus Blast when evolving). Among these, I'd pick Water Pulse for the good PP and high chance of confusion, Flamethrower for the equally good PP and high base power (even if the burn is useless), and Shadow Ball for the decent PP, solid base power and high chance of lowering SD. As for your fourth move... actually, I'd go with a STABbed move, even if it's at melee range (three ranged moves are more than enough). The two choices you have for that are Tackle with the insane ammo clip, and Body Slam with the slightly higher base power and great para chance.
I wouldn't worry too much about Dialga if you do pack a STABbed move, since STAB is worth as much as a SE hit in this game. And since NVE hits don't reduce damage all that much, you're looking at a Tackle/Body Slam that does almost as much damage as Brick Break. And there's the sweet para-hax if you pick Body Slam. But overall, X-Eye Seeds are Dialga's kryptonite, you just have to clobber it with your best attacks in the several turns it's not doing anything.
(Edit: Just in case you think base power matters... it doesn't. Damage is proportional to the SUM of the attack stat and the move's power, not their product. That means the difference between Tackle and Body Slam is made up by 6 points in the Attack stat.)
Speaking of Dialga, I went and recruited him right now... I would've liked to know that you skip the save point between Hidden Highland and Temporal Tower for whatever reason. In my first run I used it to take a ton of Reviver and X-Eye Seeds for Dialga, and just now I had to do with the X-Eye Seeds I found in the tower. Also, Roar of Time DOES hurt like hell... it missed Torchic, but took away half of my level 70 Bulbasaur's HP. Thank God there's only 4 PP on that thing.