Mob tagging
 

I want to assist my brother in leveling up his character, but cant group with him due to me being a ridiculously higher level. Aside from buffing and healing him, is there a certain % or something that I can start attacking the mob and he will get full experience and such?

If I recall correctly, he has to account for over half the damage done to the mob.

It kind of depends what class you are really.

For instance. If he is attacking a mob and the only damage that mob is taking is his Melee/spell damage then when that mob is at 49% health you can wack it till its dead and he will get xp because he has done more than 50% of the damage to it.

If he has a damage shield on and he is only able to hit the mob for 1 melee point of damage and the mob dies he will get xp, however; if in the same scenario you tried wacking the mob and did more than 1 dmg point then you would steal the kill.

Conversly, if you were to have a damage shield on and let the mob hit you till it was almost dead (by getting agro from sitting near the mob) and he came up and did 1 point of damage minumum to kill the mob then he would get the xp.

This isn't how I recall it. I thought that if a mob has 100hp, and player A does 90 damage to it, then player B does 91 damage in one hit, player B will get the experience.

It's not just 51% of the mob's hp, it's 51% of the total damage done to the mob.

Sort of. He has to out-damage you. In most cases, this simply means doing 51% of the damage, but not always:

Mob has 300 HP, your brother does 151 damage (51%), then you nuke it for 250. You win because you out damaged him. Even the over-damage counts to the claim.

More than once as a lowbie I've had people unintentionally KS me, the first time being at the OoT gargs, I had the thing to like 45% and a 'helpful' druid finished him off with whatever AE nuke he was using to quad the specs. Garg died instantly, I got no exp and I could not loot it.
Whatever damage I did, was 100%, he did 101+%

thanks guys, easy enough concept to comprehend

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Exactly. Early on, I tried to help younglings by tossing a nuke once the mob was below 50% only to get yelled at for KS'ing. It would be nice if it was that easy but on the flip side it would be yet another way to PL and there are already plenty of those.