In my world the Human Empire had always been in a constant struggle against nature itself. One of the Empires foes is a parasitic/symbiotic collective of sentient plants.
Basically treants but humanoid sized and much more nimble, with some larger specialized units sprinkled in. In defensive battles they would mostly use ambush and guerrilla tactics as they can hide themselves extremely well in forests.
In my specific case the plant based species would have:
- Mana infused bark and wood...
- ... allowing them to move them as if they were muscles ...
- ... but making them require to reroot every few hours in their home lands to recharge their mana stores.
- A node based network that functions as means of long distance communication as well as enriching the land with mana. These nodes can be destroyed to drive back their influence but are fairly bulky.
- Non-verbal collective intelligence, constantly sharing information telepathically between individuals within a 50m radius. (The collective is intelligent, the individual stupid.)
The humans have at their disposal:
- Roman empire level military organization
- Late medieval metallurgy
- No access to magic
Edit: For clarification: the collective is intelligent enough to learn and to adapt different military maneuvers over time.