Because they're coming upward towards her from the ground, and its not just one tree she's on but quite a few in the general vicinity. Not all of them but more than just two, or three. If I really wanted to hurt your character enemy archer is still in the center of the forest on the high ground.

As for the blue boy he's barely holding his own, and Lancer is not going full out on him. If nothing else she does pity him because of hero's bridemaid. The only other option was to have Saber come up, and block them himself but he's not going to show up to later. Archer shooting them down is absurd given the circumstances and insulting. The guy in blue really isn't in it for himself just as Lancer says. If I really wanted her to Lancer would kill him. Originally berserker of red was to fight Lancer but since our berserker charged in face first, and left the others out there I needed to give him, and opponent and Archer doesn't make well just pegging people from the grey castle. Assassin is arguably the weakest of the enemy servants and isn't fit to fight either Lancer, or berserker outright and it would make little sense for him to engage her directly instead of staying in the forest. And Saber would run Lancer through in a one on one fight because of his skills.