Okay I beat the game, here is how you beat it.
[Tutorial:] Combine Goblin + Goblin to make Orc, then Combine Orc + Orc to make a Troll. Drag the Goblin away from the ring to keep them from suiciding. 2 Troll can clear the level, you can drag slime to stun the enemy, if you do that well you can clear with just 1 Troll. [Note: I'm not sure if Troll available at the start, if it is not available, then army of Orc and constant slime stun can do the job]
[1st Stage: Introduce Imp] Drag Slime near the watchtower, then drag Imp to combine with that Slime to create explosion that destroy the watchtower. You can combine Slime with Slime to make them tougher so that they don't die before you can drag Imp onto it.
After you clear the watchtower, simply make Troll to deal with the enemy, while Slime + Imp can keep doing explosion.
[2nd Stage: Introduce Plant] You need to learn about 2 combination here. 1st is Range Plant, Plant + Slime. 2nd is Flying Imp, Combine Imp and Goblin to create Necromancer, then combine Necromancer with Imp to create Flying Imp.
Just spam these two and the level is done easily.
[3rd Stage:] Range plant outrange the enemy, so just spam those. For some reason if you let them be, they can walk into the range of enemy, so after you combine them, drag them manually near their attack range and put them there.
You should put them all together so you can buff them with Occult Tentacle, which is create with Plant + Imp.
[4th Stage: Lava] There is a Skeleton(Slime + Goblin) that ignore fire damage, but that is useless. Let's learn about Dark Mage, the strongest unit. First combine Goblin + Plant to get Shaman. Then combine Goblin + Imp to get Necromancer, finally combine Shaman + Necromancer to create Dark Mage, this is a very powerful unit, two of these should be able to clear this level.
[Fairy Sister] They have small range, so just spam Range Plant + Dark Mage.
[Elf Queen] This one is annoying because the spawn point is so close to attack range that most of the unit died so quickly. First of all you need to deal with one of the tower.
First combine Slime + Slime to make Big Slime and drag that slime to the bottom tower. The right side will keep spawn Slime / Imp, if you get a Slime, drag that slime to the Big Slime to make it bigger so it don't died, and when you get the Imp, just drag the Imp to create explosion. The Tower has 2 health, so you need to bomb it twice.
Once the bottom tower is down, thing become easier, just spam Range Plant + Dark Mage at the bottom left, away from the top tower. You have a lot of time in these level so the faster you bring down the tower, the easier this become.
[Rogue] You can easily beat her by using Troll + Dark Mage, once you have those two just keep drag them onto her. You can also drag slime to stun her, the faster your hand is, the easier this stage become.
[Conclusion] The game is pretty hard, and the CG is not worth it.
