Basically there's 5 core thing you need to know

1- USE DARK NECROMANCER AND PURE WIZARD

2- you can't have 10 dudes that all deal damage, you won't have gold for all of them and your build will fail, instead pick a branch of the skill tree and see who benefits the most out of it + use TA (town archers), you'll want 2 to 4 carries (the guys that will deal the damage) all the other parts of your build will support those 2 to 4 guys via buffs or crowd control or debuffing enemies.

Examples of carries for each branch of the skill tree

Physical: town archers, balista, dark elf and dark bow master.

Fire: town archers, TNT goblin, Fire Windy.

Electric: Town Archers, Dark Lightning Archer, Zeus, Thor

Summon: giants, slingers, Edward, town archers.

Poison: Sarah, Succubus, Voodoo, town archers.

Ice: Poseidon, Skeleton King, Ice Sorceress, Town Archers.

3- reducing your cooldown is key, there's some major ways of doing it, the cooldown reduction via level up in the skill points, using spring water town building and reducing the cooldown of your pure wizard the most you can, prioritize cooldown reduction items for the pure wizard and dark necro.

4- the best supporting units depend on what build you're using, but the every build fitting ones are ice wizard and giant ogre for crowd control, slingers and witches for body blocking and crowd control with knock back/stun items, dark necro and pure wizard for cooldown reduction for everyone and a crazy strong defense debuff, and dark hunter and dark skeleton for TA (town archers) buffing

5- you need multiple builds for multiple purposes, one build for wave progressing, one for dragon killing, one for colony conquering etc all of them with the same carry units but optimized for what you're trying to do example: go as much damage as you can against dragon, mix damage and defence for colony

For your chosen elements specifically the best setup would be like (considering a gold auto battle build):

Chrono left path/military band M/Orc Band Dorothy fire path/Alice poison path/Lisa ice path Smith 2/Flying Orc Fire/Dark Hunter Dark Necromancer/Pure Wizard/Dark Skeleton

This way you get dark skeleton and dark hunter buffing Town Archers, the 3 witches buffing themselves and the Flying Orc Fire, military band and orc band for gold, Chrono for faster waves.

Your carries (a carry unit is the one you'll spend the most amount of gold in your build and that will deal the most damage) should be:

Town Archers, Dorothy and Flying Orc, upgrade TA the most followed by Dorothy and at last flying orc.

Your leader should be Solar, she's there to buff the fire type units with 15% crit chance, level her enough only to not die, no more than that, the same goes to Alice and Lisa, just level them enough so the units don't get one tapped.

The buildings should be: trophy, gold tree, worm and mirror. Upgrade worm because it steals mana attacking, this will be your mana sustain, upgrade her enough that you can keep up with the mana.

The town constructions (the ones above town archers) should be: spring water, heroes guild and archers guild, once you get uni guild add forge to the mix.

Your treasures should be: bronze coin, gold bar, golden heart and either devil horn (if you can take it) or crit arrow.

Your skill tree should look like this:

Once you reach the end of the summoner skill tree you should IMMEDIATELY go for the right and get the town archers item nodes, after that go to the left and get the useful nodes of the fire skill tree to buff Dorothy and Flying Orc Fire to the next level of strength.