I am sorry but because of Recaptcha I must post from mobile (where I have free proxy only for few mins), so can't write long posts and they are full of typos, if you have questions better write on my blog.
Open MTK flash tool, change language to ENG which should there already as well in Settings/General/Download "DA DL All with checksum" should be checked. Then just choose as Download-agent MTK Allinone DA bin which is supplied together with MTK flash tool and Scatter-loading file choose alston_fddopen which is in TWRP Recovery folder. Whole process of flashing 2 times is quite straightforward, only major thing which needs to be said is that battery needs to be removed to completely shut down phone (and do this also when phone starts automatically booting after successful flashing), otherwise it didn't worked for me no matter how many times I successfuly flashed phone there always stayed original Lenovo recovery.
In this Chinese thread you have picture tutorial in first post which is selfexplanatory even in Chinese:
bbs lenovo com/a8/t402705/ (replace missing spaces with dots)
Hahah, I see the guy added also photos for Chinese TWRP for international users, since he saw my posts there.
Not sure what is possible to see wihout registration to forum. Here is my screenshot how it should look after successful flashing when you can remove battery and do it for 2nd time and then after 2nd time boot into TWRP recovery.
oi57 tinypic com/2z8tix0.jpg (replace missing spaces with dots)
Btw. yesterday I already tried Xposed, but it informed me that A806 is running probably Aliyun OS (CN crap) fork of Android so it's not recommended to install Xposed to avoid bootlops and other things, which I am not missing since it's similar situation with TCL S950 my wife has, she can't run anything higher than Xposed 2.3.1 otherwise it would end up in bootloop, but at least it seem like proper Android and not some Chinese fork.