At the time, Grom had no influence from demon blood and was just a strong but regular orc and Gorehowl wasn't imbued with any particularly powerful.
In the case of WoD, he specifically went for the head because he prepared with Garrosh, and it was also a weak point. His greatest individual feat was one shotting Mannoroth in WC3 and WoD both because he struck weak points. Grommash also was imprisoned by Gul'Dan during WoD.
The profile lists Grom as 6-C and 'stronger than Gul'Dan', despite him showing few if any legitimate feats that suggest that tier.
