At level 35. You can make a level 5 gem at begining if you have no skill points spent.
A level 5 gem is enough to solo the first few waves of every level that have under 120 starting hp.(the field you played have 64 starting hp just saying). .
To defeat shadows, by far the easisest way at lower level, is to use a level 4-5 purple gem. Wait until the shadow turn invincible and as soon as it lands freeze it. Then place a tower under it, and quickly put the purple gem with Beam.
In just a few seconds his whole armor will disapear, then he becomes pretty easy to kill with a high level gem.
How come? Shouldn't that be extremely powerful, having critical hit, chain hit, bloodbound, poolbound, supressing healing, mana leech, slowing, armor tearing, and poison (even though poison is much more effective if used in traps)?
A good strategy is putting a Black/red dual gems in traps near the begining with reloading speed, and tehn when your getting in trouble these gems will have thousands of hits and then you upgrade them and combine them into your main atatck power gem.
Cuz of the high number of hits the black power will be very powerfull and gives lot of damages.