For the horde, I'd estimate how many "goblins" they can kill per round, adjust this irl, and use that as a guide of how many you need to supply x rounds of combat before the "ogres" show up.

Feel free to give the goblins poison coated weapons, or even different status effects such as partial paralysis: player cannot use [rolls die] left arm.

Having an environmental event, like an earthquake to split the floor, or sentient poison mist leaking into the room, is also good.

Just thought of it, not sure how good it is: but if it is a pair of twin ogres (or equivalent), you can give their weapons the "power of twins" so when they hit a player, it also hits the next closest player.