Army formations matter a great deal while attacking in this game. Here are the optimal monster hunting formations that I used with 1 of each troop type with tier 1 human troops. Yes, even a level 25.
With this, you should be able to kill a level 24 monster within a week of playing on a free to play account, with no research or equipment.
Just make sure to level up your Darina (Rider) and Karolina (Archer) as high as you can as they have up to 30% extra damage against monsters. You don’t have to have the same Infantry and Catapult units as I have there. Just use Irving as your starter infantry hero instead of Doug as I have here.
Monsters Level 11 to 15
Monsters Level 16 to 20
Monsters Level 21 to 24
Monsters Level 25
Zero Wounded Monster Formation
At this point you most likely have other heroes upgraded and may have higher tier troops. Then the goal would be to reduce the wounded troops to zero.
Try coming up with your own formation. Below is the level 25 monsters formation and the formation I use for 0 healing 3 weeks into the game. If you can achieve 0 healing for level 25, then it will work for monsters of lower levels using the same formation.