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Raid issue

Anyone please tell me, why is onslaught is not immune to fire debuff against human torch in Raids ? My onslaught died because of fire damage .
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Look at Torch's kit and then look back at the screenshot you just shared:
In your screenshot you have 2 incinerates on you, which according to Torch's kit reduces your regen rate by 40%, so your regen masteries may be full, but your regen rate is currently no higher than 75%.
Next, Torch has 26 smolders on him (plus the 2 incinerates on you), so the damage of his incinerates is increased by 336%, bringing the total to 436%. Onslaught's kit reduces incinerate damage by 150%, so the damage coming through is still 286% of Torch's normal incinerate damage.
Given that this is also raids and it's a larger opponent, his attack value is higher than those Torch's you usually take with him, so he's also just going to hit harder in general.
Anyone that knows their kits knows that Onslaught vs Torch isn't the best option if it's not going to be a quick fight. There is no bug here and there is nothing wrong with the raids version of Torch. But it looks like you were able to hit cap anyway, so it doesn't really matter all that much.