It's easy if you know what to do.
True.
I already beat the insane mode 4 times in a row, it's really not that hard.
At the beginning I get the Egg turret and I upgraded the attack and range of the turret, make one Slinger and wait for the special, then you can start doing 1 Infantry + 2 Supports in the first stage, keep doing this strategy 1 or 2 Infantry + Supports(take a look at how many supports can attack at the same time), sometimes you can make Anti-armor + Support or Armored + Support, but remember they cost more, so use the common sense, try to upgrade as soon as you can the attack and defense of support units, and don't waste your money on the Income upgrade, this crap doesn't work.
And remember, don't use specials, only if you are about to kill the enemy.