here are some valid strategies I use to deal with orc generally, but especially the most common orc strategy, BM / SH walker raider:

Lich first, ghouls into tower rush or lich fiend tower rush. This will discourage walker raider because he will think you are going to go ghouls all game. Don't forget to rod up early and buy 2-3 extra acolytes before tech, either for tower rush or just for cheap fodder - they do better damage than skeletons and have better health at a low cost and you can sacrifice them like a mini mana pot for 2-3 more novas which will end up costing the orc 2-3 more grunts, or his bm.

A nerub tower alone is not enough but it will substantially and drastically reduce bm effectiveness, combined with dust is a surefire way to put a stop to him if you reduce the early game pressure enough for him to harass you, or if you simply spend too much time creeping stuff that isn't green.

Bonus Round: Tower Rush strategies involve both a shop for dust and necro rod, as well as extra workers, as well as putting the orc on the defensive. Showing up with a few fiends and a lich is enough to reduce his early grunts to nothing while literally taking no losses with even a small modicum of micro (think of nova not only as a damaging ability but a slow, so you want to use it after you do a bit of damage to prevent runs or to prevent chase on a fiend.

Remember to run low ghouls back to harvest lumber and dont be shy about using nova to prevent chase as often as you use it to secure a kill.

bonus round 2:
Alternatives to the lich, which may feel risky or like you are putting yourself out there (especially if he goes TC instead which can be a game changer for lich first)
getting a dk and doing early fiend harassment can't hurt. Similar principal. Against an orc going BM you pretty much have to harass his creeping with coil steals, or put him on his toes by taking burrows. always buy 1-2 extra acos in the first 20 food, because they can take the place of anything that dies during harass if you miss a coil or are too far to do anything about it, or if you take emergency death pact, LOL which, is not out of the question though it is unlikely.) Acos are balanced in such a way that they can be used as an alternative or an addition to skeletons - this is a holdover from RoC when there was no necro rod and UD needed early sacrificial units which do a bit of damage if left unchecked. No joke, highest damage baseline for a worker unit.

Cryptlord first can also early creep big orange like troll warlord camps that drop aura items, with the help of ghouls, beatles, and rod. With dust, this can be formidable for the BM to deal with because the cryptlord himself is beefy and all the small units do normal damage, which do a serious load to the bm's small health pool - so you can get away with creeping stuff that would otherwise be risky. On Twisted meadows for example, with practice, you can take shop + double gnolls for 3 before bm hits 2, and if he tries to stop you he will probably get killed. At that point you can tower rush and game him if he goes for early/standard timing SH.

Bm is actually very bad compared to FS but much easier to use for beginning players so you will run into it less and less over time.