Nature Structures

Phoenixes are powerful creatures, but their health isn't impressive. And their immortality is insignificant compared to that of the Void Dragons.

1 Phoenix - can easily handle levels 1-2, and with the support of your heroes, can take on certain level 3 units.
2 Phoenixes - can handle almost any unit from levels 1-3.
3 Phoenixes - can handle any unit from levels 1-3.
4 Phoenixes - can defeat some level 4 monsters.
*This is, of course, assuming you have a good strategy, and at least a 3-3-3-0 setup against level 4 units.

1 Void Dragon - can defeat some level 1 and 2 monsters. It's best to avoid Gnomes and stay away from Golems.
2 Void Dragons - the same as with one, but with a slightly increased potential.
3 Void Dragons - now it's something. You can defeat some level 3 monsters. We still fear Golems. We don't even look at Thunderbirds!
4 Void Dragons - first of all, how did you manage to gather 4 Void Dragons! Now I can kill 3 monsters! Although level 4 monsters can still give me a hard time. I'm still terrified of Thunderbirds.
*We never look at Black Dragons! We can't defeat them.
-To make them solo (with the support of heroes, of course) and defeat everyone else, you need 7-8 Void Dragons. And even in this case, the Thunderbirds will peck at us, the Phoenixes will burn us badly, and we'll have to kill the Angels at least 1.5 times, and we might run out of mana.
-Void Dragons are good in combination with a decent strike force (Phoenixes and Griffins), because then 1-3 Void Dragons (1+1+1) are enough to simply keep the enemy confused, allowing the strike force to not be afraid of attacks from all sides. When the Void Dragon is out of mana, it becomes a level 3 unit, and not a particularly outstanding one (except for its health)