Great Game, I played at a long while ago, but I noticed that the Phoenix card is still to strong, i won nearly every game by spamming phoenix holding 10 fire energy back and then casting spells to kill the enemy.
I would propose to add this to the Phoenix, " If the Phoenix dies and is reborn Fire Power is reduced by 1" If this still doesn't "help" maybe this instead "If the Phoenix dies and is reborn Fire Power is reduced by X, wherby X is the number of phoenixes currently in play by the owner."
So, @Krolan and BennyGreen already gave sometips. I also went fort the Bow/Shotgun/Cannon with the 12 archers infront of my Castle than 4 wall, 3 Lines of Spiketraps and 1 Line of Frost traps, the slow can really help but it could be just prefrence, also get the lightning spell with fully upgraded mana regen, it helps if those dragons get to close. Another reason why I took the Archer variant is because Fire Elementes are Immune to fire - duh
Also if you don't get the star rating because your castle got hit early, you should invest some points in Castle Regeneration since at the end the game checks for the Castle health not how many hits it has taken.
So, I sucked at this I restarted after wave 5 or so, and then made it till the end, I got first the arrowhead that lets you punch through armor, then the archery command, going to the best feather for accuracy, to longbow, after the longbow i never even looked at the other cross-/bows. Get the broadhead its amazing, you actually can just dont bother with the spear man not deling enough damage as its worth. the level 3 catapult is amzing get it if you can, bear its ok but not necessary. I only repaired my Gate if it was below 30% these are my stats: 384 Kills, 897 Shots so on Average 2,2 Arrows or so, but atleast 70 got through probably more only 4 Headshots but 26 to the knee, yeah and Funfact: With the average weight of an arrow with 72g, i shot away 65kg wood.
If u have difficulty getting the hard badge, play easy first get some schools and teach 5 people 10 in each skill. Finish the game they will ask u if u want to save another city, do that change the difficulty to (seriously hard or impossible difficulty). Help the new city as well with your 5 Superguys.. rate up if it helped you.