I actually wrote a guide to (most) the NP bosses a while ago, here's a link if you want to see:
www.warriorscatsgame.proboards.com/thread/1441/ultimate-guide-np-enemiesAnd here's the section on the rats:
Dirty Rats (Lv. 25)
HP- There are six rats, each with about 1000 HP.
Moves- Claw Shred, Medium Heal, Stunning Crush, Toxic Blast (Group Venom)
Recommended Level- 25, minimum is after all your cats learn Claw Shred.
Recommended Items- Antivenom, Yarrow, Pheasant/Catnip
Strategy- These rats are all the same. Same HP, moves, everything. They each have rather low HP, only 1000, which is less than even the rogue cats! However, they are very dangerous as there are so many of them. Usually, on their first turn, they will mostly try to stun you with Stunning Crush (somehow). With all those rats, all your cats will most likely be stunned. And then, they will hit you with Toxic Blast, which is basically Hyper Poison except more accurate. Again, you will most likely be poisoned. Last, they will try to finish you off with Claw Shred. The worst part of it all is that they will repeatedly use Stunning Crush again and again and again so you won’t be able to do anything except watch.
The way to counter this strategy is to pick off as many of them as possible on the first round. Claw Shred works fine here, but be sure to target the right rats to be the most efficient. If you are at a high enough level, you can pick off three or four in one turn. The remaining rats will most likely not be a problem anymore then.
Do not attempt to venom or stun the rats. It’s too risky to have a cat try a status move instead of attacking, and statuses are not as useful with the rats’ low individual HP and the lack of power in venom. You may want to save your Golden Fish for a later battle, too.
Drops- None