Oblivion Ring
is a spot removal for longer games where you need to respond to threats that your opponents have.
Path to Exile
is an alternative, but it is much more expensive ($ wise) and it only hits creatures. O-Ring can hit a wider range of threats and give you the 1-2 turns you need to finish the game. (Good in sideboard so you can swap them in, and then hold them in hand until a vital combo piece or large threat hits the table).
Hidden Strings
is an absolute beast in a Heroic deck. Why? Because when you have
Aqueous Form
+
Hidden Strings
you can trigger Hidden strings every turn. And
Hidden Strings
allows you to trigger heroic twice. If you choose, you can grant you're attacker psudo-vigilence by using one target of
Hidden Strings
on him every time he swings. Or, you could just target him twice for two +1's. But wait, there's more!
Hidden Strings
can also be used to untap your lands, or tap your opponents. No matter how you decide to use it,
Hidden Strings
grants a level of control that is admirable in an aggro deck. But again, it's a little to slow to be running in a regular match, so it lives in the sideboard.
For more defensive games, in addition to
Hidden Strings
you can use
Triton Tactics
. Due to a slower tempo, it is likely that you will have more than one creature hitting the field and being important. This cards lets you share the target love and buff multiple creatures, as well as either attack without tapping, or untap for a surprise block. While
Triton Tactics
is not the powerhouse that
Hidden Strings
is, it can still be a very useful card.
Next we have Ethereal Armour. This card is excellent. The only reason I chose to take it out of the mainboard is because of the lack of enchantments in general in the mainboard. The only enchantment that works well with it now is
Curiosity
, because
Ordeal of Thassa
blows up, and
Aqueous Form
renders the first strike useless. However, it's still a worthy card, especially if you need to be blocking.
(Just for an illustration: Putting two Ethereal Armours on grants the creature +4/+4 as well as two +1/+1 counters... Yeah). If you want to run this deck with a couple more enchantments and a few less instants, Ethereal Armour will be the best card in your deck.
Last but not least is
Lithomancer's Focus
. This is to deal with artifact decks that get around the "protection from colour" spells that make up the majority of the instants. If artifact creatures are an even bigger threat in your playgroup, I would also recommend adding
Apostle's Blessing
to the sideboard (perhaps replacing
Triton Tactics
?).