I very much wish there was some kind of consistent countermeasure. So far the best I've seen is both random alliances working together, in which the objective is no longer 'win the FL' but becomes 'stop the premade winning the FL'.
MNK stun/knockback is nice vs DRKs using their LB on cliff edges or the Onsal podium but it's something of an opportunistic attack, relying on the DRK being in a good position and there being few healers around to support them.
SAM LB might be taken advantage of against DRK/DRG/DRG/DRG premades, using the Chiten buff and then Guard as soon as you see the marked DRK coming, allowing yourself to be drawn into the voke and attacks, before using your LB to try to instantly kill one of the BH5 DRGs before they escape. That said, it doesn't remotely stop them and they'll quickly build that BH again. The cooldown is very long so it isn't something which can be spammed. It is also useless against any premade team which uses RPR, as obviously, without Guard you will die to the LBs before being able to use your own LB.
Forming your own team of RPRs and chaining your LBs may be another option: RPR LB is unavoidable (even applying to DRGs in their LB hide, I believe) and cooldown is very quick. Perhaps chaining these Hysteria debuffs might be enough to enable the DRGs to waste their LBs. That said, such an approach would require your own coordinated premade to be formed and I still strongly believe that having your own premade party should never be obligatory to be able to play FLs competitively.
I haven't tried SGE in PvP myself, but I have considered it when trying to think of ways to combat premades. SGE LB offers invincibility and you don't need to be directly stood within your group to use it. But the buff only lasts for a very short time (5-seconds?) and the CD is incredibly long. Ideally, multiple SGE or an AST would be needed to make it a remotely credible option (again - forming your own premade should never be obligatory). Also, reading the skill description it seems that any enemies within the LB area get a DoT which also allows them to attack the protected players. Whether this would work against DRGs if they cast their LB outside the buff but actually use it inside, I'm not sure. Never tried it.
The protective area can be moved to a new location after initial casting, so whether you would be able to place it over your team to give your team the invincibility buff and then quickly move it to a new location far away to prevent the premade getting the DoT allowing attacks, I'm not sure. The duration is only 5-seconds long so timing would be crucial. I haven't tried it yet personally, and I presume there is some obvious downfall that I'm overlooking, otherwise it would be more widely used.