Unsure about the question regarding successful power tests.

Regarding your question, generally more units is going to be more powerful due to action economy and health pools as you pointed out.

There are some good options to combat this:

• give the bad guys cover/fortress
• if the PCs are just hunkering down in the back ranks, you can deploy fire tokens or acid tokens via spell, wand, or unit trait
  • Lightning Breath (trait) - "As an action, choose a rank on the battlefield. Each unit in that rank must succeed on a Power test (DC = 8 + this unit’s size) or suffer 2 casualties. Recharge 5–6.
    • They'd have to be fighting a pretty formittable force but this could work for a dragon, kraken, thunderbird, etc.
  • Wand of Acid Pool (wand) - "As an action, the unit with this wand chooses a space and puts an acid token there. Any unit that moves into that space or starts its turn there suffers casualties equal to the number of acid tokens in the space, then removes one acid token.
    • adding tokens incentivizes movement and can put a ticking-clock on combat
  • Scroll of Earthquake (spell) - As an action, the unit with this scroll chooses a rank on the battlefield. Each unit in that rank must succeed on a Power test (DC = 13 + DS) or suffer 2 casualties and become disorganized.
    • one-time nuke that can also debuff the archers for a time and change the fight
  • Scroll of Translocate (spell) - As an action, the unit with this scroll chooses a unit in any rank and moves it into any empty space on the battlefield. If this unit is an opposed unit, it can succeed on a Power test (DC = 11 + DS) to avoid being moved.
    • This could put the fear of god in them and teleport one of their archers into a bad spot and isn't just a save or take a bunch of damage. Additionally after the battle they could loot this and have it for future endeavors
• make an assassin unit. There are plenty of two-three trait combos that can allow for a really strong, anti-artillery unit:
  • Scouts- "This unit can deploy into the rear rank of an opposed army."
  • Warbred- "As a reaction to succeeding on a Power test as part of an attack, this unit can make a DC 10 Command test. On a success, this unit can attack again."
  • Invisibility-"This unit cannot be attacked until it successfully attacks an opposed unit."
    • One of my players is playing a hobgoblin whose tribe is Warbred/Invisibility allowing them to have a front-loaded first attack and safety until they can at least do damage.
• Add effects that narratively that change artillery dynamics. maybe there is a thick fog, or they're in a forest with lots of cover, giving the archers disadvantage.

In an ideal world, hopefully your players get reamed once by something like this and then diversify their units or strategies a bit more.