well i guess its back to the drawing board for pixie | nixie, just out of curiosity i imagine that's what reminded you about your own card?
It wasn't. I was diving back into old cards because I was in the mood to do a few quick remakes with better art and I just happened to see this card sitting near Psionic Shield | Psionic Barrier (
http://elementscommunity.org/forum/index.php/topic,22985.0.html) (which got submitted to Crucible without a bump).
This is an on-element growth (for upped), meaning you can now have mono grow decks without even a splash. Frankly, I'm not sure how to react.
This is a good point. For balance, growth abilities require duo elements. However, when you compare the +2/+2 of growth costing 1 
or 1 
for golem and forest spirit with 2 
for sacred spirit, I believe the increase cost justifies the mono element. Though light also has pegasus, which increases the atk bonus to +4 per turn effectively when diving. Also something to consider is ball lighting + mitosis + vitalize in a duo life - light deck, which already has excellent synergy together with rustler.
My thoughts on balancing:
The unupped duo boost compensates for being able to buff other creatures by giving a weaker stat boost, while the upgraded mono version compensates with increased cost. Pegasus still requires either a trio unupped or a upgraded spirit and an air mark, which means you'll have to splash more elements or spend more upgrades for a better functioning Pegasus/Spirit deck than a Pegasus/Blessing deck.
For Spark:
Blessing + Spark costs 2 Cards + 3
to give you a 6 | 3 attacker.
Sacred Spirit + Spark costs 2 Cards + 2
1
( which roughly converts to 4
given duo basis) to give you a 4 | 1 attacker.
Overall Blessing is more efficient in the short term and gives you a more durable Spark while Spirit is more efficient in the long term but leaves your with more fragile Sparks (each successive Blessing requires 1 Card 3
for +1 attack, each sucessive Vitalize requires 1
(roughly 2
given the duo) for -2 Attack -3 HP and -1 Card needed).
