Alright so Marill - Base 70HP, base 50 Def. Not all that bulky actually
Dialga - Base 120 Atk. Physical Dialga isn't actually too shabby, even in competitive (Check smogon :3).
Anyway back to the reason why you did so little damage. At base 70 power, Slash doesn't exactly measure up as the strongest physical attack ever. Additionally, it also depends on your stats - for example, if your Dialga has low attack IVs/EVs, it's attack stat will be lower, making it do less damage to Marill.
Anyway recheck your Dialga's attack stat. And oh yeah, its nature, as if it has an Attack hindering nature, that reduces it's attack stat by a further 10%, making it even weaker.