This RU combo/aggro deck layers several instants on top of Kiln or Nivix to win in a single strike. There are many versions of this deck, and many names for it. Delver/Fiend best describes this variant.

If facing a slow, removal-heavy deck, "plan B" is to time a quick strike with Reckless Charge when opponent is fully tapped.

The most common way to stop Delver/Fiend is to take out its most effective creatures, most especially Kiln Fiend. Much of the build variation relates to how to protect such creatures or somehow inflict critical damage before the creature is removed. Having experimented with a various builds, I came to prefer defensive solutions that double as offense, as pure defensive spells can dilute the main punch of the deck.

Another weakness of this type of deck is the combo often not coming together. This is usually solved with deck manipulation spells (Ponder, preordain, brainstorm, gush, etc.). I chose more indirect methods.

DEFENSE:

The best defense is ALSO a good offense, thus reinforcing the main theme of the deck instead of taking away from it. Therefore I dispense with stuff like Pyroblast in favor of things like:

Brute Force
Reckless Charge
Apostle's Blessing (much prefer prior 2)

Brute Force is obvious: +3/+3 negates the common 3 damage spell, or can provide a huge offensive boost (or in rare cases both if the removal spell is cast on my turn as I bring it out, and then follow with a Reckless charge).

Reckless Charge is less obvious but more important: Control decks will have their way with me when they have access to mana. But control can be slow. With Reckless Charge I can possibly wait until I have enough mana for a one turn victory. That could be as early as turn 4 if I get a Kiln Fiend combined with Reckless Charge and Assault Probe, while opponent's creatures are tapped. Alternatively, if I manage to somewhat damage the opponent early on before my first creature gets removed, the second creature won't need to do a full 20 damage so Reckless charge combined with a Kiln or Nivix may be enough to end the game.

Lastly, though the main deck will have 12 creatures (4 Delver, 4 Nivix, 4 Fiend), I will address opponents with extremely heavy removal by having additional creatures in sideboard, primarily Elusive Spellfist, one Whirlwind Adept (hexproof). I avoid too much countermagic as it uses up a card that could have been a creature - trading one for one. I guess if they can nail 2 creatures with one spell that would be a reason but there's not too much that does that.

COMBO issues

I have more access to the crucial evasion piece because of flashback on Artful Dodge and Defy Gravity. Should they end up in the graveyard for any reason (I used them once, I went up to 8 cards, opponent casts a discard or mill spell, etc.), they are part of my extended hand. So I will effectively have more chances than just 6 of getting one of these cards.

Being heavy on flashback also acts as defense against discard spells. I still get to use these cards once from the graveyard. As further defense vs discard decks, I have Psychotic Purge in sideboard.

The double strike piece also becomes a little less necessary thanks to Reckless charge (which also has costly flashback). This is because I may get in a first shot for 10 damage with Reckless Charge. And then when I get a 2nd creature get in a second shot of 10 damage along with Reckless charge, though that second shot only works if I have a lot of Mana to cast the Reckless Charge from the graveyard.

The one remaining vulnerability of this deck is creatures. If I get no creatures in the opening hand, I should probably mulligan, though I might take a chance on starting hands that have everything else - Reckless Charge, Artful Dodge, Assault Strobe, and 2-3 lands (both colors). Note that Ideas Unbound partly addresses this vulnerability. If I get to turn 3 and still have no creatures, but do have Ideas Unbound, I can use one to rework the hand.

Ideas Unbound is primarily intended as a late game card when I am low or out of cards. I want to use all my cards before the end step when forced to discard 3. I am really surprised that no Kiln fiend decks use it, so far as I've seen.