Access to this encounter requires you to collect each of the Guardian fragments (Phoenix, Minotaur, Hydra, & Chimera) which can be gotten as 100% drops from each of the Guardian bosses on their respective T16 maps. Once you have all 4, you can put them in your map device in a fashion similar to the Sacrifice fragments to open passage to The Shaper's Realm.
The Shaper's Realm consists of a series of boss fights. Typically, you need a character capable of handling T16 maps and their bosses without too much trouble. The first stage requires you to fight 4 randomized and upgraded map bosses on each of the branches away from the center of the map. This will open a portal to the Shaper.
The Shaper must be fought in 3 phases. The first phase is the most basic. He can use a melee attack, throw 3 orbs at you in series, use a slam attack with large AoE, use a beam attack, or have a summoning phase in which he is invulnerable. Occasionally, homing orbs will float in from the edge of the screen. If they detonate on your position, they will create a persistent floor patch of Cold damage over time. Try to group these to one edge to avoid overrunning the small arena. The best opportunity for damage is typically when he is in the channeled beam phase. You need to avoid the targeting path, but after that you have a few seconds to get in as much damage as possible. Getting behind him is preferred. The first phase ends when he hits 50% health.
Between the first and second phases, you must traverse a randomized map area (generally, in a northeasterly direction). You will want to do so quickly because the Shaper will regenerate health over time. At the end of the traversal, you will fight a mini-boss who should be familiar if you've run any Shaper-influenced maps.
The second phase adds a new attack, Ultimate Chaos, which has Zana drop a protective shield in the center of the arena while the Shaper creates a bullet-hell outside. You don't want to be outside of the bubble as you will generally get 1- or 2-shot unless you have max spell block. You need to keep Zana alive in this phase which means not letting those homing orbs detonate in the center of the arena where she makes the bubble -- you may need to get dodgy to prevent that. Furthermore, you need to kill any summoned monsters quickly as they exit a portal to prevent them from attacking Zana. The second phase ends when he hits 33% health.
Between the second and third (final) phases, you will again need to traverse a map area and kill a mini-boss who has upgraded abilities from the last. Try not to stand too still if your DPS is on the low end. Again, the Shaper will regenerate health over time, so be quick.
The third phase adds a "shadow clone" ability for the Shaper. His clone is not damageable. You must be wary of this doubled attack pattern and choose your opportunities for attack carefully. The Shaper dies in this phase when you reduce his health to zero.