Next Academy: PCs against really bad odds.

How to fight (and win) against really Big who are really Bad? That's the question posed to anyone who wants to answer by +Douglas Cole, no matter the system. 
Since I'm into D&D Next, I figured I might throw a couple pointers in there based on my personal experience. Being both a player and a GM currently, in this post I'll see it from the PC point of view first, while I'll address the GM point of view later on.

"As you move into the room, the altar suddenly collapses on itself and the massive stone statue slowly rises from it's throne as the silver door closes shut behind you, it's black glyph glowing once again. As seconds pass in utter silence, the temperature plummets and a glacial cold emanating from the Stone Guardian makes you shiver..."

D&D Next is fairly straight-forward and very light on rules, so most things are left to GM interpretation, which is the way I like it, with the correct players. That said, here are a couple basic things, based on the rules we have for now.

Be prepared

No matter your class and background, in D&D Next, you should strive to think out of the box before hand, and be ready for things to go, brutally and fast. Having your weapon, armor, shield, spell-book and whatnot is a good start, but it's not enough, especially when fighting against something that should, mathematically, destroy you.

Stay Versatile

Make sure you have weapons able to deal different kind of damage, even if minimal. Crushing and Slashing are the two major offenders here, never leave town without at least one weapon of each. Piercing Resistance is less of a worry usually but can happen. Silvering your weapon should be the first thing you do once you get 100 gold to spare.

Range. 

There is no reason to go in melee-range until you need to go in melee-range, especially against enemies without range capabilities. Range can offer you cover, which in a system like D&D Next, is quite a big thing, the +2 or +5 AC mattering a lot even against powerful enemies. Always carry some sort of range weapon with you, period. Oh, that and because of flying creatures played by a GM with half a brain :)
D&D Next considers that you can sheathe or ready a weapon as part of another Action or your move, or both. Therefore, each turn, you can easily move and ready your bow, Attack Action then ready your shield and sword, keeping your protected should anything come at you.

Items.

The equipment list is a little light for now, but already you can prepare for some serious dungeoneering if you want to. Because of the bounded accuracy system, even normal and mundane stuff will remain somewhat useful at higher level. 
Things like Caltrops, Hunting Traps and Nets are great tools to control big daddies who might have crazy STR scores, but a DEX that sucks.
Other items such as Alchemist Fire, Oil Flask and Acid Flask are somewhat less useful overall because they do damage, which doesn't scale well. However, they STILL do Fire or Acid damage, which is just what you might need against that thing that seems immune to every single attacks you have.
Anti-Toxin and Healing Potion are simple but efficient. Carry one of the each, plus spares when possible.

Fight as a Team

Big nasties usually have a couple things going for them. Could be crazy AC, devastating attacks, powerful spells and whatnot. Here are a couple things you, as a team, can do to mitigate some of what seems unbeatable.

Against High AC:

If nobody seems to be able to hit, don't just keep rolling attacks after attacks, praying for a critical hit. It's just not working. Try something else instead:

• Help Action: Identify your biggest chance to hit and do damage or effects to the enemy, and Help him! That will grant advantage on his attacks. It's not the most glamorous action but it's better than a TPK in my book.
• Grapple: This is often your best bet against high AC but normal beings (ie: Evil Cleric in full plate with shield and whatnot). Just jump on the sucker and Grapple him down then Restrain him! That will provide a good boost to all attacks against him as well as reduce all his future DEX saves. As a bonus, you can now move him around as you wish, which might be nice if you want to just throw him down the Pit of Doom just around the corner.
• Combine your actions: Help someone who's Grappling the big nasty, while someone else is Hindering him. If no matter what you try to do, the enemy seems impervious to your weapons, if four of you jump him and help/hinder/grapple/restrain, his AC20 or immunities don't mean much any-more. Once you got him, just manacle the bugger and gag him, good luck casting your Disintegrate spells now.

Against Devastating Attacks:

Some nasties are just devastating if they can hit you. With D&D Next being what it is, these are usually far more dangerous than the High AC ones.

• Keep at range if you can: If the nasty can one shot most of you in melee-range, stay as far away from it as possible! To do this, spread out and combine various actions with your regular move such as Dodge (if you get cornered), Disengage or Hustle. Soon as possible, try to figure out the creatures move per turn and if possible, stay out of reach and spread out. If he goes toward one, the others can pelt him at range while the current target focuses on getting away.
• Stay close to your Protection Specialist: If you don't have a Protection specialist, it's very risky for anyone but the best armored guys to be in range. If you have one however, even that AC14 barbarian can get in there, at least for a turn or two, benefiting from the Disadvantage he'll be able to offer. A clever Protection specialist will have grabbed the Feat Tactical Mastery, gaining even more control over the battle. If he's an experienced AND clever one, he'll have also the feat Lucky (for those lucky hits) and Shield Mastery (for spell saves), which makes him a true pillar of defense to build your strategy around.

There's probably plenty more that one could do and all in all, it comes down to some dice rolls, but that should be a good start.