Ah yes...those. If I could give them one idea it would be to kinda make it like objectives in League of Legends... for instance killing those bosses would grant timed buffs that would boost either combat or speed in taking other objectives or shorter cooldowns on spells. Something that would be strategically best to do in order to make these boss objectives appealing and worth it.
For example, a group is getting ready to lay siege to stonemist castle...they would find it a lot easier to go kill this or that boss in order to get cooldown reduction or siege speed buffs for everyone (with a duration of say 15-20 minutes. The people defending would find it wise to also send out a team to get the buffs, even just to ensure the attackers dont get it.