Good point. But please remember, the above diagram is static. In the actual game the rats were all over the place (and are probably under-represented above). Any openings we had were shut down because we didn't prepare actions in time or weren't valid options (in one instance, my gnome could have bull rushed, but it would have failed against the large creature automatically). I very seriously doubt that 6 players at the table actively looking for bull rush opportunities would have missed an opening like that.
You did miss it, seriously, as I already explained in my first post. It goes by simple logic that three rats cannot block all six opportunities to Bull Rush the Oozes.
You likely forget two things:
1) You can attack diagonally, and as such also Bull Rush diagonally.
2) Push does not mean "away in a straight line" but "each step is further away than the previous".
Ignoring the rats for a moment, you had the following opportunities to Bull Rush:
1) PC @ E7 - Bull Rush attack Ooze Sr @ D6 - push one step to west
2) PCs @ E5-7 - Bull Rush attack Ooze Sr @ D5/6 - push one step to north-west
3) PCs @ E5-7 - Bull Rush attack Ooze Sr @ D5/6 - push one step to north
4) PCs @ E6-8 - Bull Rush attack Ooze Sr @ D7/8 - push one step to north
5) PCs @ E6-8 - Bull Rush attack Ooze Sr @ D7/8 - push one step to north-east
6) PC @ E6 - Bull Rush attack Ooze Sr @ D7 - push one step to east
Each two adjacent allies help via the Aid Attack action, timing is done by readying.
If you need, go back to the board and rules to validate that:
1) All above moves are legal.
2) You need four and not three rats to block all the above moves.
Beyond that there are many good suggestions about a creative solution, which a sane should have allowed in on or the other way. ("Say yes or roll the goddamn dice !")
What's wrong with polearm defenders and human TWF rangers? In any case, my gnome rogue was multiclassed into wizard for some blasting potential and we had a Gnoll Warlord for healing too. Sadly, our warlord got eaten by the Blue Ooze and so I didn't feel a need to mention her for this scenario. (I could still hear her death rattle upon closing the doors!)
1) You're almost exklusively melee. (Does not matter in this case)
2) You lacked any positioning powers.
3) Your defenders (two !) took worst of two worlds - low damage and no shield. Reach needs a bit beef-up before it is good for a defender (excepting a Polearm Gamble Fighter).