If you're not going to breed, there's still some improvements you can do to your Aggron. Lets analyze the pokemon's stats and learnset.
As we said before, Heavy Slam is Aggron's best Steel type STAB, hitting most pokemon with 100 or 120 power, which is awesome for a move with 100 accuracy, no drawbacks like recoil and 10 PP.
Ancient Power is mediocre in many reasons. First, it's a special move, so it will use Aggron's mediocre special attack (60) instead of its attack stat, which is much higher (110). Besides, it has 5 PP, which is not desirable at all in story mode since you will have to fight many times before you heal your pokemon, and you'll run out of PP very easily. Finally, Ancient Power has 60 power, which is mediocre at best. If we look at Aggron's learnset, we can see that it learns both Rock Tomb and Rock Slide. They are both physical, so they will hit harder, and they have more than 5 PP (10 and 15 respectively). Rock Slide has higher power and hits both opponents in double battles, and Rock Tomb lowers the opponet's speed, allowing Aggron to outspeed it after one or two hits normally. You should seriously think about changing Ancient Power for one of these.
Iron Defense is not a bad move, but it has a serious drawback. Suppose you are fighting a physical attacker and you get to use Iron Defense twice, getting a +4 in defense. It's great, you're immortal and you defeat the other mon easily. Okay, what if the next pokemon is a special attacker? Your Iron Defenses will be useless and you will be forced out losing all your setup. Besides, Aggron doesn't need more defense. With a base 180 defense you are able to defeat any physical attacker with no problem without any need for boosts. A more useful move for this slot would be Metal Burst + Sturdy, since Sturdy makes you able to survive any hit and Metal Burst will kill any pokemon afterwards.
Finally, you should know that Normal and Dragon are awful types for coverage, so Dragon Claw and Strength make no sense at all. If you want a coverage move, Earthquake will be the best option when we get the TM.
Another possible set for Aggron is running Autotomize or Rock Polish, which doubles your speed. Just remember that if you are running Autotomize you need to play Iron Head over Heavy Slam since Autotomize halves your weight, so Rock polish is prefered.