Naxt Victim

How to summon the specific bosses is very simple. Each different Sentinax Beacon summons mobs that drop additional Sentinax Beacons for that specific Beacon and ultimately the Portal Stone (boss).

You get the Uncommon (green quality) Sentinax Beacons from the standard mobs in the area, you then need to farm the mobs that come out of the Sentinax Portal until they drop a Rare (blue quality) Sentinax Beacon and use it to empower the portal, it is only after this that the Portal Stone can drop. Keep using all the Rare Sentinax Beacons you get to extend the duration of the Sentinax Beacon.

If you already happen to have a Rare Sentinax Beacon you can use that immediately, the portal will spawn regardless if there's a portal spawned by an Uncommon Sentinax Beacon or not.

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• [npc=120686] requires [item=146909] -> [item=146915] -> [item=146921]
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• The mobs that spawn from these portals are [npc=120684], [npc=120685], [npc=120682].
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• [npc=120681] requires [item=146908] -> [item=146914] -> [item=146920]
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• The mobs that spawn from these portals are [npc=116928], [npc=121928], [npc=116913], [npc=120679], [npc=120678].
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• [npc=120675] requires [item=146907] -> [item=146913] -> [item=146919]
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• The mobs that spawn from these portals are [npc=117034], [npc=121927], [npc=120666], [npc=120674], [npc=120669].
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• [npc=120665] requires [item=146906] -> [item=146912] -> [item=146918]
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• The mobs that spawn from these portals are [npc=120663], [npc=117211], [npc=121024], [npc=121922].
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• [npc=120641] requires [item=146905] -> [item=146911] -> [item=146917]
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• The mobs that spawn from these portals are [npc=121894], [npc=116986], [npc=116988], [npc=121921].
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• [npc=120583] requires [item=146903] -> [item=146910] -> [item=146916]
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• The mobs that spawn from these portals are [npc=120579], [npc=120578], [npc=120575], [npc=120576], [npc=120577], [npc=120580], [npc=120582], [npc=121893].
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While it may not be up all the time, the Nether Disruptor building is a fantastic way to get the greater beacons used to grind for the epic quality beacon to summon the bosses. Each of the rare mobs that spawn from the unstable nether portals, with use of Nether Portal Disruptor, have a high chance to drop a greater beacon. They will usually drop uncommon beacons of the same type which can then be used to extend the duration of the greater beacon.

So this is not as hard as it first might seem. We did three bosses in an evening pretty fast and pretty efficient.
What you want to do is:

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2. Make sure to read "Xirev"s amazing comment on how the beacons work.
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4. Make sure the "Nether Disruptor" building is up on the shore. This strategy depends on it.
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6. Get a group of 5 people. Be sure to have a tank or healer for the bosses later.
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8. Get some "Nether portal disruptors", sells of one of the Nethershard vendors on the shore. Get about 4-5 each.
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10. All fly around in a group looking for "Unstable portals" on the shore (they are marked with a purple icon on the mini-map).
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12. Activate the portals, and all loot the boss spawned, he should drop several "Sentinax beacon of Greater XXs"
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14. After you got a ton of these (and you are out of Nether portal disruptors) get to under the Sentinax.
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16. Follow Xirev's amazing comment about what beacons drop what boss item.
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18. Use one type of beacon at a time. I.e. start of with all the groups "Sentinax beacons of Greater torment" for example. Use one beacon every 10th second or so. When someone loot the epic boss beacon, call it out and all stop using rare Greater beacons.
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20. When the portal is cleared, summon and kill the boss. (Beware, some of them might be hard) After he's dead, repeat from step 7, picking a new type.
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22. If you are all out of Greater shards from that type, without getting a boss beacon (highly unlikely) you either repeat from step 7 if you got more beacons from another type or from step 3 if you are all out of beacons.
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If you after one or two iterations of this only have one or so bosses left, do check Xirev's comment of what beacon type you should target to get that boss to spawn.
General tip is to communicate and keep it structured, so it is best if the leader gives clear directives on what beacons to use and when to do bosses and so on. Very easy that people jump the gun and start spawning a lot of different type portals and that just becomes a mess.

ed. updated the username credit to Xirev!