They were not able to withstand (ouk ischuon antistēnai). Imperfect active of ischuō, to have strength, and ingressive second aorist active (intransitive) infinitive of anthistēmi. They continued unable (without strength enough) to take a stand against. Stephen knocked them down, Saul included, as fast as they got up. Stephen was like a battery charged and in action.
The wisdom and spirit (tēi sophiāi kai pneumati). Dative case. They stood up against Stephen’s wisdom and the Holy Spirit “by whom he spoke” (hōi elalei). Instrumental case and the relative agrees with “Spirit.” He kept on speaking so (elalei, imperfect active). It was a desperate situation.