What should be the coil ratio to wirelessely transmit 12V/8A?
This has been discussed before, see:
Inductive wireless mobile charger circuit   

Overall I suspect that trying to efficiently transfer 8A will very difficult.  

For other references see:
http://www.low-powerdesign.com/120706-article-wireless-power-charging.htm   

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1136&amp;context=eesp  

Here is a design you can experiment with:
https://www.mepits.com/project/171/DIY-Projects/-Wireless-Power-Transmission-Mobile-Charger-Circuit-Using-Inductive-Coupling