Grunts heavy armor. So magic attack does 2X damage on them. This is why casters(in this case : Sorceress) are used by HM.
HM normally goes quick tier 2 -> Double Arcane Sanctrum. Hero AM 1st, MK 2nd. Sometimes quick tower rush when Sorceress come out, somtimes normal play.
Orc normally goes 4 grunts, Raiders, Spirit walkers with 1 upgrade. Hero BM 1st, SH 2nd. Sometimes FS is 1st and TC is 2nd.
Sometimes orc uses Wind riders. This is kind of 'new' 2 counter casters; and can be countered by simply taking Beastmaster as 2nd hero by HM. Water elemental and Quilbeast are enough 4 wind riders. But WE and QB can be 'disenchanted' by Spirit Walkers.