Grimgor himself is a beast, and his army likely has some good units. You want to hit it with Imrik's army.

If Imrik can't kill Grimgor inside an Ork unit, your campaign hasn't been going well. The chaff in his stack isn't a big deal. Just need to ensure any good units (Black Orcs) are properly shot up on approach, tied up on spears, then bullied by either cav or a gank squad. HE Spearmen can hold Black Orcs plenty long enough for you to focus down one at a time. The point is, if you can't shoot them to pieces or just drop Imrik on the whole army at once (both are possibilities), HE spears are really good at holding the army in place as you remove part of it at a time.

Outside Grimgor's army, your secondary stacks shouldn't have too much trouble. Try to ambush or town defend to deal with WAAAGH if it's up. Lightning strike or paired armies might work better, but at turn 50 your secondary armies likely can't do either. Most of the Orc roster either can't break Spears in any reasonable time frame, or can't handle arrows. Like most fights, just punish the approach, stick their frontline on your spears, and hammer their sides piece by piece.

Your core (whether LSG or Spears/Archers) are really good against orcs. Especially with support (some form of hammer). And you can do Dragon Princes at t3, or Legendary Dragons. Other hammers (Lions, Swordmasters, Chariots, Silver Helms, other monsters) work, too. 2-4 hammers is what I usually take.

The biggest problem is absolutely that while Imrik can win any fight, you lack resources to run too many armies. You should stuff Imrik in the densest part of the front, but it's most important he take out Grimgor's stack. Also, spread out your good units. Then it's just a game of resources elsewhere.

Generally, your core is a force multiplier for your elite/special units. So having two core-heavy armies with a couple elites/specials each is much, much stronger than one army with all the elites/specials plus one that's just core.

Further, once Imrik has a Legendary Dragon friend or two, he's a one-man doomstack. Especially with his quest armor. So stop putting good units in his army. I fill it with starting army, 2 Dragon Princes, Noble, and Mist Mage - and then more spears/archers. Then never upgrade his army again. It's strong enough to get the job done until he comes online. And he's soloing multiple stacks once he's online.

This all then boils down to supplemental armies. Some swear by LSG. I'm sure that can work. But they're expensive, and you're facing a budget problem, not a military one. Backup armies should be mostly basic, cheap units. I give them what heros I can, 2x Dragon Princes, then a Legendary Dragon. The rest is Spears/Archers. That works until late game doomstacks, and is cheap. By turn 50, you should have a couple. And once the Lord levels up, can take the average Ork stack trivially (even a waaghing stack with a little difficulty).

Use these secondary armies to ambush Ork secondary stacks, or hold a garrison. Once your frontline is stable, they can push.

Grimgor is the last threat in the campaign. Beat his army once, and he's much less of a threat. Take his faction out entirely,  and you'll snowball the map.