601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can’t be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.
Mana cost: 2G
Additional cost: 1G
Cost reduction: 2
Total cost: 2G + 1G - 2 = 1GG
Yes.
Notice the plural in rule 115.9b; having multiple targets does not invalidate the cost reduction:
115.9b An object that looks for a “[spell or ability] that targets [something]” checks the current state of that spell or ability’s targets. (...)
And costs increments are calculated before costs reductions:
601.2f The player determines the total cost of the spell.(...) The total cost is the mana cost (...), plus all additional costs and cost increases, and minus all cost reductions.(...)
If i have a Elderwood Scion and a Fabled Hero on the battlefield and i cast Colossal Heroics on both of my creatures, will the additional cost from Colossal Heroics be reduced when targeting Elderwood Scion?
What about if i cast something like Tandem Tactics and simillar effects? Will Elderwood Scion make the spell cheaper?
The total cost will be reduced.
Mana cost: 2G
Additional cost: 1G
Cost reduction: 2
Total cost: 2G + 1G - 2 = 1GG
If the spell you cast targets your Elderwood Scion (and there is generic mana in its cost to be reduced), absolutely.
Notice the plural in rule 115.9b; having multiple targets does not invalidate the cost reduction:
And costs increments are calculated before costs reductions:
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules