But if your opponent is ready with a Regress, a Vendetta, or a Heat Ray, then that creature-enhancing Aura will cause you to get seriously set back as your opponent two-for-ones you and develops a large tempo advantage in the process.
So what should you consider when you are deciding when it will be appropriate to cast your Auras? When should you draft Auras? And is it possible to build a deck around them?
What to Consider
When they are allowed to resolve and live without being harmed, (many) creature-enhancing Auras tend to be very powerful cards. The problem with Auras is that when the creature that they are, or would be, enchanting gets destroyed, bounced or in some other way removed from play, then you are in for a world of hurt.
Getting two-for-oned—when your opponent uses one of his or her cards to deal with two of yours—tends to be fairly devastating. Getting two-for-oned might not be that bad if you are say, trading two of your underwhelming Glory Seekers for a potentially menacing Wildheart Invoker. But it can be especially devastating when your opponent deals with two of your better cards at once without spending much time to do so.
Yes your opponent took 4 damage to destroy your creature, but for the mere price of one mana your opponent was able to deal with two of your cards and a total of eight mana worth of investment (two mana to cast the Beastbreaker of Bala Ged, three mana to level it up and three mana to cast Snake Umbra).
Unfortunately, it's hard to play around a card like Vendetta or Regress with your Auras. The strength of these cards is that they are just so cheap and so versatile that they make it very difficult for other players to maneuver around them effectively.
But that doesn't mean you can't play around removal spells with Auras.
You can cast your Auras early in the game and hope that your opponent hasn't drawn an answer yet. This is especially easy and profitable to do with Auras that have totem armor because even if your opponent does have an answer, so long as it isn't something like a bounce spell or an Induce Despair, or he or she can't cast it in response to you then you wouldn't have to worry about getting two-for-oned as your creature would get saved by totem armor.
If your opponent ever taps out, that is a clear opportunity to slap down a large Aura with totem armor. If you notice that your opponent has, say, three lands, including a Mountain, open, then you would be a lot safer putting your Aura on a creature with three plus toughness than you would be putting it on something smaller so that you don't get blown out by a Staggershock.
But if your Aura does not offer totem armor, then you have to tread a bit more carefully. Even if you find your opponent tapped out, you might not be completely safe from getting two-for-oned, because your opponent would be able to simply untap and cast a banishing effect on one of your creatures.
Thus you might decide that you would be best off if you waited until you drew, or had the mana to cast, an answer like a counterspell or an Emerge Unscathed.
Or you might wait until you get well into the game before you cast your Aura(s), anticipating that your opponent will have already used his or her answers.
Even though it might be more effective if your opponent has nothing to put a +3/+3 enchantment on one of your creatures somewhere during the early-middle stages of the game, it is typically much safer to spend your turn casting a reasonably sized creature if your opponent has mana available that he or she could cast an answer with.
If it's turn five, you might want to put down a Drake Umbra to defeat your opponent with quickly, but if your opponent is amassing a fearsome attacking force and you are worried that he or she might have an answer to your would be gigantic flyer, then you would probably be better off just casting that Wildheart Invoker to safely advance your board position.
To sum things up: You can play your Aura early, and try to beat your opponent quickly before they draw an answer or have the mana to cast their answer(s), you can wait until you have a way to protect your key threat with something like a counterspell or an Emerge Unscathed, or you can wait until your opponent has used all and/or tapped low and doesn't have an opportunity to cast his or her answer(s).
Your Opponent's Creatures
Fortunately, it's much simpler to use Auras on your opponent's creatures. It is pretty much always a fine time to cast a Guard Duty or a Narcolepsy on one of your opponent's creatures. Aside from counterspells, there are hardly any ways that your opponent can stop a Pacifism type Aura from affecting on one of his or her creatures with a card or ability that he or she could not choose to use at any time.
A card that gives a creature protection from a color, such as Emerge Unscathed, would be able to remove an Aura at any time. A Disenchant effect, such as a Demystify, would similarly be able to remove an Aura like Lust for War at the drop of a hat.
When Should You Take a Totem Armor Aura?
A card like Boar Umbra is powerful enough that it is well worth a first pick. If you are on the play and you spend your third turn putting a Boar Umbra on even something as innocuous as a Glory Seeker, or a Nest Invader, you will be attacking with an incredibly scary 5/5 before your opponent has even had the chance to add anything relevant to the board.
Heck, even later in the game, giving one of your creatures +3/+3 and totem armor can be enough to swing things fully in your favor. Functionally, a Merfolk Skyscout becomes a dragon and Rapacious One becomes something that closely rivals an Eldrazi monster.
Cards like Hyena Umbra, and even Spider Umbra tend to be quite good because they provide a very relevant effect for a very small mana investment, however they are simply not powerful enough to warrant an extremely early pick. You should feel free to take a Hyena Umbra around 3rd-4th pick on if there isn't anything else that is particularly meaningful in the pack, but if your deck is slow you might want to wait a bit longer on the cheapest Auras.
Eland Umbra is a reasonable 20th-23rd card, which I wouldn't mind taking late in a pack, but Crab Umbra is barely playable and not something that I would want to take over anything that is even decent.
Auras as a Strategy
Waaaay back in Ravnica Block Limited, my favorite strategy to draft was a green-blue deck that was based around Skarrgan Pit-Skulks, evasion creatures, power-enhancing Auras and creatures with graft. The strategy used an unpopular color combination and a lot of unpopular cards, especially the commonly scoffed at Skarrgan Pit-Skulk, to put together decks that were actually quite powerful.
My draws with the deck would typically start off a bit slow, and then in the course of a turn or two I would vomit out a bunch of cheap and heavily enhanced evasion creatures that my opponent would usually have no way to deal with.
While Skarrgan Pit-Skulk wasn't very impressive on its own as a 1/1 or a 2/2, when you grew it to 4/4, 5/5 or more, it would become a force to be reckoned with. All for the low, low price of a single green mana.
With the aid of some bounce, removal, or simply some giant growth effects, your opponents would be in for a world of hurt at the hands of the surprisingly strong Skarrgan Pit-Skulk.
We see this happening again, but on a much larger scale, in Rise of the Eldrazi Limited thanks to Aura Gnarlid.
On its own, Aura Gnarlid is not a particularly noteworthy card. A 2/2 for three that can't be blocked by creatures with less power than it would be playable, but it wouldn't be anything to brag about for most decks. Fortunately for you, and unfortunately for your opponents, Aura Gnarlid is very rarely just a 2/2. Especially if you choose to consciously draft and build your deck around it.
The second that you put a Hyena Umbra on a Aura Gnarlid you have a 4/4 first striker with totem armor that is incredibly tough to block. If you then put a Guard Duty one on of your opponent's creatures, you suddenly have a 5/5 first striker. And if your opponent then puts a Drake Umbra on one of his or her creatures in order to try to swing the race, you will have a monstrous 6/6 creature that is very tough to block. If you then put a Mammoth Umbra on your Aura Gnarlid you would have a nigh-unblockable 10/10 first striker with vigilance. Ouch!
While that is undoubtedly a good sequence of plays for you, and it's fortunate that your opponent didn't have an Oust or a well-timed Vendetta to ruin your day, it's important to note that there is nothing unreasonable about that sequence of plays. It is even easier to conceive of less explosive sequence of plays, such as just casting the Hyena Umbra, that allows your Aura Gnarlid to become a fierce threat.
Aura Gnarlid is one of the few cheap, combat-oriented, creatures that you can play that tends to remain relevant throughout the game without any direct mana investment.
Sure a level up creature like Caravan Escort will be quite a force once you've invested a total of eleven mana in it, and a Dawnglare Invoker will need to be killed once you reach eight mana, but that isn't the same thing as having a creature that requires a meager one time investment to be a force at pretty much any point.
Once you've gotten into the second or third packs, and you know that you have/are going to get a good number of Auras, then it is completely reasonable to take Aura Gnarlid with one of your earliest picks. Similarly once you have a couple of Aura Gnarlids and a couple of Auras, you will want to take things like Hyena Umbra, Eel Umbra and Spider Umbra (depending on your colors) eve n earlier than you would normally.
Just keep your curve low, and the Auras flowing and your deck should turn out to be very powerful.