Or you could pin people in place. A small group can draw much larger groups merely by threatening areas, because blobbers want to dominate battlefields. As long as you're okay with losing some people on occassion, multiple fronts can allow for a hell-for leather opening.
Sure, you'll lose all the troops that rush the gap, but who cares (if you're Arcaea)?
During that time thier clock is running and your clock isn't, though Clocks only run if you think of armies as blobs rather then cycling groups. Anyway, I just know how it would likely run from the Arcachon end with the players reverse, but I will point out you can't object with both "Bah, we need better marhsals" and "We have thought of every tactic you lot of thought of, because we have awesome marshals."
Why would we need excellent marshals to think of tactics? I'm quite good at tactics myself though most of my training was for RL armies it is often applicable. I make a pretty terrible marshal due to my activity times and because of my characters RP attitudes.
If you recall much earlier in the war I did run a little raid group into Arcachon and lead a chase for a little while. One of the problems was with the geography of Arcachon it is quite easy for the defenders to box in raid groups, thus the raid group must be large enough that Arcachon can't afford to split their armies to catch you, which of course leads to the fact that you are probably now removing quite a sizable portion of our active nobles from the armies in order to go raiding, which limits how effectively the armies can capitalise on the effects of the raid anyway. Arcaea actually isn't too bad at running their armies as separate groups, you may recall we had to do it quite a bit while we fought wars on both borders.
But you pretty much hit the nail on the head as to why your "superior" tactics for Arcaea aren't applicable, they rely on the activity and abilities of the current Arcachon group. Arcaea currently has a player group with different strengths and weaknesses, and thus needs to utilise tactics applicable to that. Its sort of similar to saying an army from say Australia should approach a tactical situation in the same way as an American army. its just not going to work.
