Aura spells require a target when they are cast. Elderwood Scion’s cost-reduction ability will affect Aura spells cast targeting it.

Elderwood Scion’s cost-reduction ability can’t reduce the colored mana requirement of a spell.

Elderwood Scion’s cost-reduction ability can apply to any cost of a spell, including additional or alternative costs.

None of Elderwood Scion’s abilities affect abilities that target it.

Spells you cast that target Elderwood Scion will cost less to cast even if those spells have additional targets. The same is true for spells your opponents cast costing more.