The actual role of preoperative embolization of brain arteriovenous malformations (AVMs) is undervalued. The present study aims to describe the effectiveness, safety, technics, and results of the endovascular-based functional downgrading of brain AVMs. Data regarding 31 Spetzler-Martin (SM) grade III AVMs that consecutively underwent a combined endovascular-surgical treatment were reviewed. Clinical and radiological outcomes were evaluated according to modified Ranking Scale score (mRS) and postoperative angiography, respectively. Low-density Onyx was used in all cases, and the timeframe between the embolizations was 10–15 days. Procedures were 1.6 (±0.6) on average. Superselective nidal catheterization was essential to reach the deepest parts of the AVMs, which were the targets. Onyx made the nidus compact and easier to dissect. The Average obliteration rate was 29.6%, with negligible morbidity. Surgery was performed after 3.7 days on average and the AVM exclusion was complete in 83.4% of cases. An mRS score between 0 and 2 was reported in 77.5% of patients, while the best outcome was achieved in small and medium-deep malformations. An effective and safe functional downgrading of brain AVMs must be based upon specific technical key aspects, which make surgery easier and, ultimately, allow for the achievement of the best outcome.