The key to much of Wargroove's strategy is a rock-paper-scissors game of unit vulnerability. Spearmen are super effective against cavalry, for example, but vulnerable to archers. To win, you'll need to maintain a mix of unit types and exploit these vulnerabilities to your advantage.
If you press B on a unit, you'll see what it's effective against and vulnerable to, but it's a bit hard to read on-screen. It might be worth printing off a copy of the chart like this one for easy reference during play.
Actually using it effectively is one of the hardest things to learn in Wargroove. As a general rule, it's a good idea to have the bulk of your forces made up of Spearmen, with some ranged support. That combination is cost-effective and works well in both attack and defence.
The reason for this is that it makes it easy to leverage Spearmen's critical hit ability, activated when two such units are next to each other. Several units have such abilities: dogs, for instance, score a critical if the target is adjacent to another unit.
Critical hits can turn the tide of battle in your favour, so make use of them whenever you can. Spearmen aren't very fast, so before attacking, it's often worth moving one unit into a space adjacent to the one you want to attack from. Then move your attacking unit into place and cross your fingers for that critical. You can rotate units in such a formation as they get damaged.
Range and Terrain
To get that adjacency bonus with your Spearmen, you need to make sure they can move into the right spaces. If you select one, you'll see it's movement spaces displayed. If necessary, cancel, select another and make sure they can all get to where they need.
If it's important to you, it's important to the enemy, too. If you hover over an opposing unit and press A, you'll see where they can potentially move.
The game gives an advantage to the attacking unit as they'll damage the target unit before it can attack back. So you should be checking range often. Not only to make sure you can get where you want but to stay out of enemy movement range, ensuring you can get in the first attack.
When planning range and manoeuvre, also take terrain and blocking units into account. You can't move through enemy units. So if you position your troops carefully, even low-health units can block a foe from reaching a vulnerable target like some Archers.
You can use terrain which slows down movement like forests and mountains, in a similar way. But terrain has a second advantage, which is defence. Where possible you want your troops on forests and mountains because they'll take less damage. Similarly, you want to tempt enemies out of such defensive positions if you can.
Over the long term, the efficiency of what you build in Wargroove becomes a critical factor. There are two kinds to take into account: gold and build efficiency.
Cannon-fodder troops like Swordsmen and Spearmen are very gold-efficient. If you pit a single expensive unit against its equivalent cost in cheap troops, it will almost always lose. That's because it only has a single attack versus the many attacks a swarm of units can inflict.
Likewise you can use such a swarm to move, capture and block choke points much more effectively than one big unit.
However, cheap units are not build-efficient. Each barracks you control can only build one unit per turn. So if you've got a lot of gold and few barracks, it still makes sense to buy powerful troops with low gold-efficiency.
Remember also that some expensive units offer extra advantages. Archers are very squishy for their cost, but their ranged attack is very powerful. Airborne units are even more extreme since many ground units can't attack them back at all.
Getting the balance right is difficult, but it depends on the map and the number of villages and barracks you control. If you can get stable income, Knights offer a great balance between gold and build efficiency and make good mid-game units. Just watch out for those Spearmen!