Build your defenses against an epic boss
You have to build at least 8-10 towers, and many trenches. create armor reducing (purple) and shocking (cyan) gems, combine them up to grade 2 or 3 and place them in most of the towers. Create a few high grade gems. A strong poison gem can also help.
Try to make pure or dual, or maybe 3-colored gems. Gems can't have more than 3 specials, so if you have a gem with 5 different ancestors, only the first 3 specials are kept, the remaining 2 are discarded. Pure and dual gems also get bonuses to their minimum and maximum damages, as well as their firing speed and range. These bonues can be boosted further with the pure and dual gem mastery skills.
The faster you kill a monster the more score you get for it, so it's better to build your defenses near the monsters' base.
You can buy high grade gems or create many low grade and combine them. The second option costs more mana, but gives you a better control on the final gem. If you have enough mana and you are going for raw damage, buy a high grade gem. Its purity will give it some damage bonus.