Tanking bosses

Let herc attack for 10 seconds, that should keep aggro. And if you can't keep aggro from sin, then you don't enough dps. I currently have +4 GV's and I keep aggro from a sin with +6 Ghost Thorns. Also tell the sin to use subsea strike instead of sparking if the sin really takes aggro a lot.

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To the OP, we are talking a BM, right? And what level?

Bms often tank if we have fists and -int. If a sin is out DDing you your options are to either get more DD or let them tank and amp them so they get more heal from bp. If they can't tank, even with you HFing and debuffing, then they need to manage their aggro. We only have one aggro skills, Stream Strike, and it isn't worth anything. Continuing to attack for more dmg is a much better option than using SS.

So, to try and tank yourself avoid HFing the sins sparks, instead triple spark yourself and ask the party to debuff/amp around your sparks, not the sins. The Sin should play support instead if he plans on you tanking, which means rib striking and subseaing your sparks.

If the Sin insists on over DDing and trying for aggro let them have it. They either die and you continue to tank, or they tank and you support them. MSS will weaken the boss attacks so they can avoid being one shotted. If they are not likely to be one shotted then go ahead and HF/Glacial Spike/genie skill so they are healing themselves with bp and DDing to the max. Can also try to interupt boss attacks for them.

All else fails, talk to your cleric and squad and let them know that only 1 tank will be healed, even if aggro changes. That normally backs sins off a bit knowing that if they steal aggro they won't be healed and may miss getting the boss.

Not much you can do if its a pure dex sin. Sin will just need to learn to tank if they aren't going to learn how to DD properly.