When you get couple of levels start with two best treasuries (+loot, +magnet), you can allow to buy best weapon after 1st boss. after 2nd boss buy +fire rate, at end sell treasuries and buy flags so you have almost best rate, dmg and medicore speed :)
For the last level: at start max one green and yellow. Max yellow and buy only greens, no need for other. At end I bought three yellows, just to be sure I wont fail ;)
Not bad game, hell hard at beggining, but gets easier when you got the pattern.
My opinion about skills, hope it helps:
ORCS: Worst of all groups, invest only for +firerate
Gold vault - good. Height advantage - good.
Cut down study - crap. Markmanship - crap. Bounty Hunting - Awesome!. Warchief - crap. Poison - crap. Mining - good/awesome if not gonna try to get brilliant. Cut down mastery - excellent. Lich king and freeze - excellent. Soul Vault - excellent. Frenzy mastery - good.
UNDEAD:
Mana stock - middle. Mana pool - Awesome. Frenzy study - very good. Mind Control - Totally OVERPOWERED!. Mana extraction - good.
DEMON - training excellent, others also marvelous. Avoid only scorching treasure and return portal ;)
To deal with ninjas: Build temples! Their area of effect attack can kill them easily with 1 shot when upgraded enough. Just need them to deal 100dmg at very first NINJA WAVES, and 200dmg at later ninjas.
To deal with Ninja Champs: As was written before - fully upgraded meteor. At lvl 14 I put Temple at heights in both corners and Crypts at the 2 others. Also always sell your low towers to not make him smoke to early. before casting meteor let your high dmg tower hit him fist.
To deal with Assasin (dodgers): Use frenzy skill, 99% none of them will survive.
General tips: You can easily beat half of game using only crypts, just upgrade them to have 3 bullets, cheaper upgrades and fear, fear, fear! It rocks.
Few maps that will left, use Red ones, I didnt use their weakening upgrade, since it last only as long enemy is in range.
In my opinion Dens (green) sucks a lot.
Remember to always put tower on mana pools, you gonna die without it.
Boss is verty easy, farm the earlier level for max hp, bfg. And remember: bullet time is your best friend!! You can easily avoid any bullets. For me it's better than berserk.
 "Arkandian Revenant"){kind=icon}
 "Frantic Frigates"){kind=icon}
 "Tactical Arena"){kind=icon}
 "Little Stars for Little Wars: Players Pack"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Death vs. Monstars"){kind=icon}