Hello everyone, I read the posts and didn't find how to deal with the Nagabanks, so here's an example for variety:
Minimum Nagabank: 2 Royal Nagas and 8 simple ones.
We have Shakti, level 3, with advanced attack and 118 Trogols. Ideally, there should also be harpies and troops from 3 heroes, but without them.
Speed: The buffed Nagas go first (7), then all our Trogols (6), and the simple Nagas have a speed of 5.
Placement: One Trogol each, the main stack under #2.

Round 1.
The Royal Nagas don't reach the main stack of Trogols, they attack the central unit #4.
Unit #1 takes retaliatory damage from the Royal Nagas (remember, a simple Naga deals 20 damage, and a buffed one deals 30).
The main stack of Trogols attacks the Royal Naga, dealing 117-352 damage, with a high chance of killing it. And it positions itself at the bottom so that the Nagas from the upper left corner can't reach it.
Unit #3 moves to the left edge so that the Nagas from the upper left corner can't reach it, one cell away.
Unit #5 runs to the upper right corner, exposing itself.
Unit #6 waits.
Unit #7 runs to the lower right corner, also exposing itself.
The Nagas from the upper right corner attack unit #5.
The Nagas from the lower right corner attack unit #7.
The Nagas from the center right flank and from the upper left corner move towards the Trogols.
Trogol unit #6 runs to the lower right corner.

After round 1.
A stack of Trogols stands slightly below the center, to the left, and to the left of it is unit #3. Closer to the lower right corner is unit #6.
Stacks of Nagas are in the upper right and lower right corners.
Another stack of Nagas is to the right, 2 cells away from the Trogols, and the 4th stack is above and to the left of the Trogols.
We will attack it now.

Round 2.
The main stack of Trogols runs to the left of the Nagas and attacks. They manage to kill them.
Stack #3 runs up the map.
Stack #6 runs as far to the right as possible.
The Nagas from the upper right corner move vertically down towards unit #6.
The Nagas from the lower right corner attack unit #6.

After round 2.
A stack of Trogols stands in the left part of the map, with unit #3 slightly above it.
Next to them is a stack of Nagas, and two more stacks of Nagas are far to the right.

Round 3.
The Trogols attack 2 Nagas. Unit #6 waits and runs to the right. The 2 remaining stacks of Nagas move to the right.

Round 4.
The main stack of Trogols runs to the lower left corner, and the unit stands at the top of the map.
The Nagas move to the right.

Round 5.
The Trogols attack the nearest stack of Nagas.
Round 6. The Trogols attack the last stack of Nagas.

The losses are minimal if the attacking stack of Trogols eliminates 2 Royal Nagas in the first round (which means all the single Trogols are lost). And if only one Naga remains, then the losses will be about 20-30 Trogols, maybe even more!!