The guys you are describing sound like the typical barbarian enemy of Rome but tougher. If they have tough hides then they are no different than a tall human wearing hardened leather which the Romans were built to fight. https://www.youtube.com/watch?v=7-cS_ptYF9I
Remember most battles were not fights to the death. The Romans had good discipline so they didn't break much but most of their enemies wanted to charge in, overwhelm the Romans, then live to tell the tale. Many battles that the Romans fought went something like "two sides meet, Romans throw waves of <PERSON> at the enemy, they get into a melee battle, they are not doing too well against the Romans, the enemy doesn't want to die anymore so the charge dies and they retreat with about 10-15% losses".
With your Orthaks they would be giant and easy to hit ranged targets but hard to hit in melee combat. 358
I would have a front line with 1-2 shield bearers with short spears designed to attack their ligaments and finish off fallen Orthaks. Outfit them with Pilum too. Behind that I would have 16ft spears stabbing at their chest, face,, and just generally being scary. Behind them I would have dedicated <PERSON> throwers since they are so nice, tall, and easy to hit. Behind that crossbow men to take full advantage of their range. I'd bring Ballesta too so anything that gives you trouble can be skewered.
If you can inflict enough damage early on then you can demoralize an enemy that otherwise has a numbers advantage.
The absolute best tactic would be the Hun strategy of run around them in circles on horses, avoiding all melee combat, and pelt them with arrows till they stop moving but that makes for a boring story.
Is there anything they are allergic to or that can weaken them that can be mass produced and ranged weapons can be dipped in?
If you know little about ancient battle tactics then these three channels will get you into ancient era warfare and show you how to make a realistic world:
https://www.youtube.com/channel/UCv_vLHiWVBh_FR9vbeuiY-A
https://www.youtube.com/channel/UCMmaBzfCCwZ2KqaBJjkj0fw
https://www.youtube.com/channel/UCwO-UgquohXwoe7f0e6lMnw
Could probably copy and paste some tactics and only historians would be able to tell. 358