Honestly, Earthquake would be better for every type of battle with your Excadrill. It's a STAB, unlike Rock Slide, and it has 100 Base Power, and 100% Accuracy, compared with the 80 Base Power and 95% Accuracy for Drill Run. If you were thinking about Drill Run because it raises Critical Hit chance, it can also learn Slash and Shadow Claw, which do the same thing. For singles, Excadrill can sweep with Earthquake if it's fast enough and/or has Swords Dance or something. For doubles, you can hit both of the enemy Pokemon with Earthquake, which would work if your second Pokemon uses Protect, or has the Levitate Ability/is a Flying Type. And Iron Head would be better than Rock Slide - it's more reliable (100% Acc. vs. 90% Acc.) and is a STAB, unlike Rock Slide.
So, a good Moveset for Excadrill:
Adamant/Jolly Nature (+Atk.-Sp.Atk OR +Spd.-Sp.Atk)
- Swords Dance
- Earthquake (STAB)
- Iron Head (STAB) +30% Flinch Chance
- Shadow Claw/Slash (Base70; Acc.100%) +Crit. Chance
Hope that helped. :)