[Squirrelloid]

Sombre, re: assassins:
(1) The lack of access to assassins is easily remedied. Introduce a spell which summons an assassin. Now everyone has access to them.
(2) If you don't have sufficient assassins and you have access to them, and your opponent pulls this tactic, that's sort of your own fault now isn't it? Especially if you let them get away with it turn after turn. The point of counters is you have them if you plan on having them, not that knowing about them instantly negates some particular tactic.

Alternate counters to blocking chaff armies set to retreat:
(1) cast CotW, GR, or similar on their province. Assuming PD, your summoned army will actually get to fight, and you can kill as many as you can. If you win, your army advances as planned.
(2) use stealthy armies or fliers to attack their originating province (and other potential retreat provinces) and cut off their retreat, limiting your inconvenience to one turn.