There are a lot of good answers here, to put it simply, do not engage in melee combat. This is a summary, plus bits from my own mind:
* Train everyone in the use of ranged weapons
* Train their stamina to outmaneuver the Orthaks. You wanted it to be impossible to outrun them, so don't, just dodge. It will take a lot of energy and stamina for these big creatures to stop or slow down enough to change direction. They will tire out quickly and become exponentially easier to deal with. Split them up in battle, don't let them form groups that can gang up and kill soldiers trying to dodge them
* Invest in making your raged weapons bigger and more powerful
* Invest in making your soldiers better with them
* Build heavily fortified positions, the Orthaks seem too dumb to siege
* "the Orthaks can run at speeds approaching that of a horse", but not matching or exceeding it. So just ride horses and you can outrun the Orthaks indefinitely. Shoot bows from the horses. Not even a single casualty
Their physical advantage is simply not enough alone to guarantee them victory, and additionally you've added a handicap by making them not very intelligent creatures. In fact, they'd be lucky not to be the dominated ones.
However, despite specifically saying that you are looking for a tactical advantage for the humans, if you are keen on forcing this melee conflict to happen the answers you are looking for are
Shock Troops and Ninjas
Shock Troops
Shock troops are frontline soldiers, they are better trained and better equipped than normal soldiers. 358
The way I would use them is to form squads of an arbitrary size, say 5. These men would be the biggest and strongest around with heavy armor, heavier than equivalent plate armor; weighing at least 80lbs. They would carry battleaxes or perhaps poleaxes rather than swords, swords are meant for humans; our meat cuts easily. You'd want and need the extra force that comes with a heavy axe to cut through tough flesh. The tactic I would use is send out fast units, the fastest horses and an unarmored, or lightly armored rider, meant to harass and piss off the advancing vanguard by filling them with arrows. They will charge and the light cavalry will retreat and spread out to split them up. At this point the shock troops will rush in and gang up on the individual or small groups of Orthraks and simply overrun them with muscles and steel
Ninjas
I don't think I need to explain what ninjas are. The tactic is maybe an hour before daybreak, you send them in to silently kill as many as possible. When day breaks, they will flee. During the main battle you will hope that the ninjas killed enough that the archer forces can dispatch the rest 358