Generating enmity from the target enemy means doing something to get real hate, such as attacking or even missing (this is opposed to merely aggro/link hate, which is a different beast). I highly doubt it means being at the *top* of the enmity list and thus actually getting targeted.
The only issue is that the original question mentioned multiple THFs *in a party*, so the answer is applicable to that. While that's all well and good for one party, that doesn't really solve alliance issues, much less multiple alliance issues such as Dynamis. Does it extend across alliance as well, or even across multiple alliances, or is it based on the enemy itself and whether that enemy has been attacked by a THF period? If it's not based on enemy, then is the applicable party/alliance decided by who gets the killshot, or by some other means?
The safest thing to do is to put a TH3 or TH4 THF in every party that will be getting killshots and have each of them tag every mob that their party will be killing (for example, have the THFs in BLM parties tag statues and the THFs in melee parties tag beastmen in Dynamis), but that may be overkill. Or it may not.