Impervious Defender

Iron Horde

• Assault on Mok'gol Watchpost
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• Assault on the Iron Siegeworks
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Shadowmoon

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• Assault on Darktide Roost
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• Assault on Pillars of Fate
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• Assault on Socrethar's Rise
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• Assault on Skettis
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Ogres

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• Assault on Stonefury Cliffs
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• Assault on the Gorian Proving Grounds (no missive available)
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Botani

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• Assault on Lost Veil Anzu
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• Assault on the Everbloom Wilds
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Goren

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• Assault on Magnarok
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• Assault on the Broken Precipice
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Shadow Council

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• Assault on Shattrath Harbor
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• Assault on the Heart of Shattrath (no missive available)
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Platinum rating is 1300 points, rewards [item=122163]

Successfully did this achievement on Alliance with a 685 ilvl mage. I would rate the difficulty of all 6 types of invasion from easiest to hardest like this: Goren > Botani > Ogre = Shadowmoon > Iron Horde >> Shadow Council. The point mark to aim for at the end of 7min 30s stage is 1105-1110 which the final boss will push you right past 1300 for platinum.

Some tips i personally used and would recommend (For alliance):
Preparation and Consumables
[item=120347]: Costing 60 GR, This restores 10k hp and mana per second over a small radius of 8 yds. Overally minor buff as mobs are no longer threatening enough to groups going for platinum that the hps is required. However, the mana gain is a fairly big dps boost to Arcane mages.

[item=120349]: Also costs 60 GR, this provides a sprint after killing enemies, universally useful for reducing downtime travelling around the garrison.

Blessing of K'ara: Zone wide SMV buff obtained from Pilgrim's Shrines found all around the zone. Procs Blessing of K'ara with fairly low uptime. Overally you want any dps boost you can get. The closest ones to garrison are at approximately 32,30 and 25,22

Bodyguards: Likewise, bodyguards are small dps gain that should not be overlooked.

Follower patrol: This is only available to the leader whose garrison is being invaded. Followers on idle wander around the garrison and can be requested to come on patrol. They provide different utility from dps to buffs depending on their abilities. I find that the best followers to bring on patrol are shadow priests as they are able to learn Timed Battle abilities, granting them the ability to buff you with Power Infusion. It grants 50% haste and has a high uptime, thus is a major dps boost. Alternatively, mage followers with timed battle ability also provide a similar dps buff, but can benefit the party if within range, and less effective (35% haste), approximately 1min icd.

Dps Cooldowns: This is obvious but dps cooldowns should generally used to improve kill speed or simply complete bonus events more quickly. In general you want to use hero near the end of the stage where mobs yielding more points start to spawn.

Mob Density Patterns:
Invasions on Alliance side are based around mainly 3 hotspots, the garrison entrance, center fountain and exit towards mines/herb garden. For different types of invasion, one of the hotspots will generally have high mob density than the others, which is where your strongest dps should be situated. As the fountain hotspot encompass a large area, range dps are most ideal for the center hotspot to minimise travelling downtime. These are the mob density patterns i observed in general:
Botani, Goren, Ogre and Shadowmoon invasions mobs tend to have more concentrated around the fountain, Iron Horde and Shadow Council mobs are more concentrated at both mine exit and garrison main entrance points.

I wish you luck getting Platinum during a Shadow Council invasion. You'll need it.