Same restrictions as the others where on her own a single blast would only be at most 3ft by 3ft and a chain on her own would be 10f by 5ft. Now, if she has another person then it would be possible to create larger chains, but each person can only reach that same max. So unless they can chain quickly they may need more people. But it's not impossible to clear the oil all at once! It's just the wall is very large and the oil thick, not to mention what is cleared will only stay that way for 30seconds to a minute max.

Even if the wall manages to be completely cleared at once, it won't stay that way. If they manage to succeed with more people joining in the group burst (either directly through people using their magic or through other devices and such), they will notice the oily substance seep through from the cracks within the walls to instantly cover it again after the time limit is up. However, they will notice that it is thinner and easier to clear in subsequent rounds.

There really is something strange to this oily substance...