Once you reach the midgame, raw health becomes the backbone of your run. This strategy is built around stacking as much of it as possible while letting thorns and orbs do the actual killing for you.
Health is the priority in the midgame. Buy it whenever it is available, regardless of how steep the cash and coin costs climb, and keep up with the health lab research as well.
Your kills come from a few key sources. Thorns should be fully maxed, since most enemies will die by hitting you rather than being shot. Orb speed should also be maxed, and shockwave helps by shoving enemies into your orbs. Equip the Inner Orbs card, plus Death Ray if you have it.
For cards, the essentials are Health, Coins, Extra Defense and Free Upgrades. On top of those, Slow Aura and Range are strong picks because slower enemies spend more time in orb range and get hit more often. Plasma Cannon or Energy Net are also worth considering: the aim is to soften bosses up enough that a single thorns hit finishes them instead of needing two.
When picking perks, grab the perk requirement options when they appear, then prioritise Health, the -50% enemy damage / -50% tower damage trade (which effectively doubles your health pool), and Defense %.
Once your health is fully maxed, you can start layering on offensive cards like Attack, Crit Chance and possibly a Super Tower, and put more into Death Wave. The point is not to switch to an attack build — it is to push your health even higher, ideally to absurd, game-straining levels.