Ch 521: The Last Danger Zone
As the swamp was full of sticky mud, walking normally was very troublesome here. So Elena was using a barrier as her foothold while walking and Tomoe was cladding her feet in lightning to get rid of all the mud around it. They could have just flown in this phase but they decided to walk for the time being.
That was mostly because this part of the swamp was still not that bad. Later in the swamp was going to get very smelly and in that area both Elena and Tomoe were planning to fly. So as walking in this swamp was an interesting experience, they were enjoying it for the time being.