That is exactly the idea here. Issuvi is a capable warrior; she should be more than capable of handling herself. That adds so much more spice when we see her fall into complete hysteria from a single finger dragging across her arch and heel. Her ticklish feet are very much so her 'Achilles Heel' hence the title of the picture. Once that finger teases Issuvi's bare sole she looses her mind; all rational thought escapes and all she can think is 'how much it tickles!' She simply claws at the dirt and screams her head off. More lashing out at nothing rather than trying to escape.