All I can say is it does seem level-dependant. At least for the 2nd hydra, while my level 50 pawn breaks out almost every time, just about, if I bring along level 1 pawns for the other two slots, they get swallowed just about every time. If you're talking about the very first hydra encounter, all I can say is try to bring along level 1 pawns, all melee types (so they stay closer to get grabbed, and generally can't assist in breaking others out as often as ranged ones).

When it comes to the archydra, though, that one rarely has trouble with my main pawn, so it shouldn't be much work to get it to gulp a few pawns in short time. X3