As stated several times already, the main issue is the overall strength of your gems. Dual red-lime is the go-to gem for pretty much the entire game, and on this pylon you'll need to kill enough monsters to build it up to at least grade 9 (with the appropriate amplifiers surrounding it, of course). Make sure you've put this beefed-up gem close to your orb, because the shadow will like to hover very close to it and launch its mana-stealing projectiles at point-blank range.
Also, you'll want to have a second tower core loaded with grade 6-7 or higher (if possible) gems. This is where the poison comes into play.
No matter how strong my gems are (i put 1 red/lime lvl9, 1 red/lime lvl7, 1 poison lvl 6 and 1 purple lvl 6), the Shadow stays right hover my orb and send its creeps straight inside it. There's no chance of killing them.
Not sure why, but I have always found it more reasonable to stick with the red/lime mix, and a pure purple on the edges. Also, just saying, but I have found having your mana pool to 75k or 100k is best. Plus, drop 6 tier 1 gem bombs on the initial start to get a much needed boost to the mana pool to get you going. Otherwise, fastest mana gain combo in the game is tier 2 yellow with a tier 1 orange and lime combined to make a tier 2. then combine to tier 3 for a triple pack. Kills fast, hits multiple enemies and generates mana. Its what most use in the reruns when they are going for the 5m or better scores on all the levels.
