You can now restore your Ascend Mate 7 (MT7-L09) from the new Marshmallow update back to stock Lollipop firmware using the official transitional firmware, build B500, released by Huawei.
How to Downgrade Ascend Mate 7 to KitKat Firmware [L09]
Here’s a step-by-step tutorial on how to downgrade your Ascend Mate 7 MT7-L09 from Lollipop (B308) to KitKat (B137SP03) using the official downgrade package provided by Huawei. After downgrading to B137SP03, you’ll be able to upgrade to the latest Android 5.1.1 Beta we recently published on this site.