Note: ONLY works in DLC
Forgotten field is probablu the easiest

sample below:

You can finish the game on Easy
Note: It does take some time but you can fast forward the game to save time
Here is the screenshot:


Once you have finished all levels go back to do any leve where you don't have 3 stars in. It should be easier now because you have upgraded spells and towers.

I think the easiest way to get this is to freeze a group of enemies and use artillery on them

The fastest way is to get this is to play ONLY on easy
You only need to get 3 stars on all levels for 1 difficulty.
See the sections below for more tips if you cannot beat a specific level

You need to max out all the upgrades for ONLY 1 spell
I recommend the freeze one because it is the most useful spell and has only 2 upgrades

Tip 1) The best combination of action skills are the hammer and freeze
You can freeze enemies as a panic button to prevent enemies from advancing
Use your hammer when all or most of your turrets can benefit
Here is an example



Tip 2) what if you CANNOT beat the level?
You can pay gems to instantly access your skill points. In this level I paid gems to immediately gain my ability to freeze enemies so that I can get 3 stars
On the right hand side you can pay gems


Tip 3) Pay some gems to gain access to the best locations in the map

Tip 4) Which upgrades are good?
The blue (heavy cannon) is generally the best unupgraded
Upgraded towers: Blue double barrel , Yellow tower with poison green upgrade, Artillery with rocket upgrade
Flying enemies are best dealt with by the Blue tower fully upgraded with the double barrel . You can use the sniper version also for long distances. However, The Yellow tower with the poison upgrade is a cheaper and safer option. It can act as a good support to freeze enemies in place

Tip 5) When you know in some levels you get rushed by flying enemies, build the yellow tower and upgrade it fully