Do allies help with slaying monsters?

Yes there is! It’s the same boost as when attacking players.

 

Yes. Allies definitely help with attacking monsters in biomes. The research that you have to do in biomes that say e.g. "Badlands Ally Bonus vs. Players" raises the bottom number under the big green number.

In the Ally Section (check your Waritorium),
The big green number is how much of a bonus you have. The "Base (Biome) Bonus. That number is the bonus you'll get attacking monsters/titans.
The smaller greyish number right under it saying "Max Bonus vs. Players" is the number you'll raise by doing the research I mentioned that's in the biome tree. That is the max bonus you can get against players you attack.
If that number is bigger than the green number, the green number is the bonus you'll have attacking players. If it's smaller than the green number, you'll only be able to get the max bonus you researched, but you CAN increase that number by researching it.

So...yes, you need allies to hit monsters more easily. The same allies help with attacking players, but it's limited by research if your allies go past the max bonus vs players.