First of all, you should remember the names of the 144 (!) Halfbloods faceable and how their decks are built.
Here's an article on how to do that.
Read that, consider the following statement: if a card from a certain element is a threat, if it is the element of the prefix it is more of a threat than the suffix one. If it is both prefix and suffix, it is almost sure that the threat is in the deck.
This is because the deck has more cards from the prefix element than the suffix one, so it's more likely for the card to be in the deck if it is in the prefix instead of the suffix.
Let's review the elements, then:
EntropyDiscord can screw you, if it's likely (either prefix or full name) for a Discord to be played early, you may want to set up a Phase Shield ASAP in order to prevent scramble and build a nice quanta pool. The only other threat is Butterfly Effect, but it's not much of a problem.
DeathMind the damage, it's a fast element. Poison bypasses shields, too, but you should be able to outdamage it, expecially if you back up with a shield. No real threat aside that.
GravityMomentum and Momentumed creatures can screw your shields. Just don't rely on them too much and go for the fastest kill you can manage. Black Hole may be annoying, but no real threat.
EarthYou may quanta starve due to Earthquakes, expecially if paired with Darkness. With Gravity, Pulverizer has the power to destroy shields and pillars, but this isn't a probelm, given that you should already be hurrying the game up. Against Earth in general, hurry up the game, shouldn't be a problem at all.
LifeGenerally known as a fast element, that's not the problem here because when it is built by the AI, most of the times it doesn't get the decks focused on rushes or something in particular. The real trouble here is Emerald Shield. If the AI plays Emerald Shield, unless you're going to lose instantaneously, you shouldn't quit. Luckily for us, Life has 2 Shields and it values every shield (aside mirror shield, if I'm not wrong) better than it and tends to change it as soon as possible. Don't use SoW when the AI drops an Emerald Shield. If you want, you can change playstyle to an Essential Halfblood Farmer and don't use SoWs at all until the AI plays a shield which is not Emerald Shield (and Mirror Shield, if it's paired with Light).
FireIf the AI got lucky with deckbuilding, this definitely is one of the rushiest elements. Deflagrations may screw your Phase Shields and Fire Bolts totally bypass it. The creatures can (and will) get really big. However, how disastrous this may look, I only once had troubles against Fire, and it was when I got only a Tower in my starting hand and it got deflagrated. Go for the rush, but don't keep the Shields in your hand too much, late game they'll be risky because AI tends not to play many Deflagrations on Towers and may have kept them in hand. It's better to stop little damage than no damage at all.
WaterWater is, most likely, the easiest element to fight against. You can play however you want, you should both be able to outdamage it and stall it, if you want. Ice Shield may be pesky for your un-SoWed Immortals.
LightThis is pretty similar to Life, refer to it to know what to do against Mirror Shield (and remember that Light, also, has a second shield). It has another threat, though: Sanctuary. Sanctuary gives both healing and blocks our SoBras. However, Light's damage output is really small, you should have more turns to play around.
AirAgain, not much of a problem. Unstable Gases, though, may need you to hurry up the game, expecially if the opponent managed a Air Nymph onto the field.
TimeHalfbloods rarely get 6 copies of the same card, and in that big of a deck it is even hard for them to draw many copies of them, but a Sundial chain is, surely, your biggest threat here (deckout after drawing with many SoBras). Not much of a threat though. Remember Procrastination when summoning Immortals without a SoW on them.
DarknessI was tempted to call this paragraph "
Darkness Devourers" due to Devourers being what makes me having Nightmares about Vampires who Steal your Dolls [/jokes].
Really, Devourer is the main problem, however Liquid Shadow may be pesky. Try not to waste quanta on useless stuff (that other Immortal wasn't really needed, was it?) and keep it for when a Liquid Shadowed Black Dragon approaches and you want to drop a Shield. Try not to make them draw too much with SoBras due to the threat of drawing into Steals and Devourers.
AetherDamn, you wouldn't expect this. The real trouble here is Mindgate. Psions are annoying, but the damage isn't too much and you can definitely outdamage that, but if your hand isn't awesome (quanta-wisely), he may copy your immortals and SoWs and, instead, have quanta to play them and DEFINITELY outrush you. However, if you start right and/or the AI doesn't draw into Mindgates early, you'll most likely win.
Nigthmarish Combinations:Earth-Darkness: mainly if it is Shadra, because a couple or Earthquakes are more than enough if paired with a whole family of Devourers.
Rush Element and Light: if a rush element gets Sanctuary, you could be done for. Also, Mirror Shield is a major problem that way. Try to deal with it using Phase Shields to stall it out.
Earth/Darkness-Entropy: it's a nightmare. Go with Phase Shields ASAP because you'll need to build your quanta pool before the opponent manages a couple of Earthquakes. Unless Darkness is the prefix, you shouldn't fear Devourers, but Steals, instead.
