The present explanatory sequential mixed-method study, taking a comprehensive approach, investigated the effectiveness of self-regulated learning-based instruction and regular writing instruction on academic writing skills among university English as Foreign Language students with different self-efficacy levels. It also delved into the students' voices regarding the effectiveness of self-regulated learning-based instruction and regular writing instruction. A total of eighty students were scrutinized in a 14-week experimental study. For these aims, forty students in the experimental group received self-regulated learning-based instruction intervention, and another forty students received regular writing instruction. Forty out of eighty students were selected using purposive sampling to obtain the qualitative data. Self-efficacy questionnaires, argumentative essay writing tests, observations, writing diaries, and semi-structured interviews were utilized to gather quantitative and qualitative data. The quantitative data were analyzed using SPSS 29 through a paired sample t-test, while the qualitative data used a thematic analysis. The results of quantitative data showed that self-regulated learning-based instruction and regular writing instruction effectively enhanced academic writing skills (contextualizing, summarizing, and sourcing) across university English as Foreign Language students' high, moderate, and low self-efficacy levels. Results obtained from observations, writing diaries, and interviews contended the benefits and challenges of self-regulated learning-based instruction and regular writing instruction.