。
She was lost in thought about the bullying directed at her.
Literal
She [topic-は] herself [against-に対する] bullying's matter [over-で] [intently-じっと] was-thinking-deeply.
~に対する ('toward, against, regarding') is the formal modifier — 'the bullying directed at her.' じっと is a 擬態語 (state mimetic) for stillness and intent focus, often paired with verbs of mental concentration. 考え込む is a compound verb (考える + 込む 'enter into') meaning 'to think deeply, brood, sink into thought.' Past progressive 考え込んでいた describes an ongoing past state of brooding silence.