Stroph, along with Kelly, were watching the battle. The girl was looking at Sibren and Erza's battle against the other Phantoms, and Stroph observing Marcus intently.
From the outside it might have looked like he was just eating chips, which in principle he was, but he was also analyzing Marcus' every move.
He watched their fight and tried to figure out if it was all true or if Marcus was faking it and he would lead them into a trap in the future. Stroph was thinking of that option as well.
To make a staged battle with hard to win, to gain the credibility, and then to use it to kill them. Not a bad plan. A possible one, but pretty ordinary and uninteresting.
The chips in the bag were getting fewer and fewer, and Marcus and his opponent were getting more and more tired. They were breathing heavily, and began to move noticeably slower.
Creation is hard, cheer me up!