Marche's BSB is an atk/mnd boost- I think that's currently unique in GL (Aemo from Dimensions 2 has one on her medica BSB if we get it), so it'll stack with everything. The commands aren't powerful on their own, but C1 is a 3-turn 15% Knight damage buff, and C2's Sentinel can buff guardbringer's hit count. Marche is one of 3 characters with a doublecast Knight LM2 (Dorgann and Beatrix the others), so if you were to commit the motes, you could lifesiphon into the BSB, then C2, C1, Guardbringer x3, and likely get at least one extra guardbringer cast in there. Subsequent BSB uses could skip C2 and just do the knight buff. Agrias could use either her burst commands, or when it comes in the TGC event, Assault Saber for more holy blasting.
It's a workable burst, though the lack of enholy hurts it.
In other news,
redditor Krissco has managed to mostly-solo Sealion with Galuf. I don't know if this is more or less impressive than when the same user soloed boundless Nemesis with Galuf.