17 Veteran Guardian Squads
Attack: +25, Counter: +25
22 Veteran Seeker Squads
Attack: +20, Counter: +30
4 B2 Super Battle Droid Groups
Attack: +25, Counter: +25
2 CB-3 Droid Groups
Attack: +20, Counter: +30
1 BX Commando Droid Group
Attack: +30, Counter: +20
14 Kaleesh Warrior Groups
Attack: +35, Counter: +30
1 Dwarf Spider Droid Group
Attack: +35, Counter: +15
8 Wookiee Warrior Groups
Attack: +20, Counter: +25, Survive: -10
16 Abyss Agents
Attack: +20, Counter: +25, Survive: -10
1 Kygeetu San Torgh
Attack: +35, Counter: +40, Survive: -20
1 Dani Plenar
Attack: +35, Counter: +40, Survive: -20
These are all the highest scoring units. I included Grievous's bonus to the Kaleesh on their scores. Unsurprisingly, the Abysswalkers are ridiculous, but we'd need to balance their inclusion with the fact that they're named characters who can actually die here. Their bonuses make that unlikely, but it's still notable. (As an aside, Kygeetu would be insane if Ciaran were here, too. Like, holy shit, outright the single best unit.) Also unsurprisingly, as long as Grievous is on the field, the Kaleesh are the highest rated non-named characters.
It seems to me that the best score to focus on is attack. Counterattack helps to resist the counterattack, of course, but if more of them are dead, then there will be fewer of them to counterattack. Survival bonuses help them survive if they fail the counterattack, but counterattack helps them avoid the death roll to begin with, and attack helps them avoid the counterattack.
Edit: Also, the more killed each round, the faster we can shut them all down each level, so that ends up with fewer counterattack rounds as well. Attack is just the best stat.
With that in mind, I'd recommend:
14 Kaleesh Warrior Groups
1 Dwarf Spider Droid Group
1 BX Commando Droid Group
12 Veteran Guardian Squads
That leaves two slots for either two more Veteran Guardian Squads or the two Abysswalkers, depending on how lucky we're feeling.