I don't know if there is a guide or not but only 476 finished the game (99 stars), made me think there is not one. And be gentle this is my first guide and English is not my native language, so some typos and bad sentences may be in here.
Tips for normal and hard levels.
*Use your upgrades only on characters and spells that you use (read the descriptions).
*If there are lot of scattered enemy's DONT USE FOCUS
*Easy way to kill jumpers - don't use focus (let your archers to the work).
*Always kill enemies with specials first
*I strongly recommend snipers and headshot skill.
*Unlock extra spots soon as possible.
Using Skills.
*Always plant mines and traps as soon its possible near you troops -last line of defense.
*Gas should be used near enemy entering areas - making massive pile of enemy's and then use Curse(let them fight each-other ), meteorite or arrow swarm (quick kill).
*Skills reload quick, use them. Dont wait until your overwhelmed and getting enough amount of kills with skill give you achievements and extra skill point.
*Use freeze on large squads, let the next squad walk in the same place and use skills: Curse, meteorite or arrow swarm.
Tips for Challenges (only thing little hard in this game)
Level 1 - Kill Shamans first, rest is easy
Level 2 - mines and traps near your troops. Gas to hold them back for meteorite and etc
Level 3 - Kill Witches first - They use the skill like in a rotation, so watch out for that and know where to strike.
Level 4 - Shamans first, freezing and cursing helps a lot.
Level 5 - Wait until you have all 3: gas, traps, mines. Always plant don't wait.
Level 6 - Like level 3, watch out for that rotation. Kill specials first by zone at a time.
Level 7 - Trust your troops to kill some, don't try to get everyone. Dont drop meteorites close enough to damage your troops, but close enough to stun your enemy's. Use gas to make large pile of enemy's for a massive kill
Level 8 - Gas to slow down enemy's and meteorite and swam to kill them all.
Level 9 - Shamans and elder Shamans first then Undead Rangers by zone (Kill the top group first then the middle and etc).Revived enemy's waite a little before they shoot, kill them quick
Level 10 - Traps and gas near enemy entrance. Lighting for ones that got away
Level 11 - Look for the rotation again. Bombers first (Shamas are gone heal pointless troops for a while) then Shamans, Witches and Undead Rangers. By zone its easier.
Level 12 - Traps at side. Use skill troop to take the focus. Rest should be common sence
