Mehmed is a Conquering commander that is designed to destroy cities in Rise Of Kingdoms. You can obtain Mehmed in gold chests. You can use him for field battles but because of Mehmed's talent tree and skills, he is pure tier S+ commander for rallying cities.
Mehmed is great commander, pure tier S+ for attacking cities but for field battles he is not that great. If you are planning to use Mehmed for field battles then upgrade only his 2 first skills. The rest of skills are only for attacking cities.
If you are free to play or small spender I would not recommend that you max Mehmed or invest in him. Leave that to pay to win player. You have a better commander that will bring you much more value in ROK.
Before you start investing in Mehmed just think about how many times you will rally cities and is it worth it? If you are a big spender then you must have Mehmed on max skills. It will make your life easier and you will be able to burn tier 5 cities with no problems.
First skill-this skill will give you an insane amount of damage when you add all other skills. They make Mehmed insane nuking commander for attacking cities.
Second skill-this skill will boost your first skill and nuking skill form your secondary commander.
Third skill-10% chance to do additional damage when you are attacking cites. If you have luck you will do an insane amount of damage in a short amount of time.
Fourth skill-you can send more troops and your rally will be around 2.5 million if you have all max. More troops in rally = more damage.
When it comes to pairings you can use any nuking skill damage commander that you have. But I would recommend that you use YSG as primary and Mehmed as a secondary commander.
Together Mehmed and YSG will do an insane amount of nuking damage and you have an anti swarm system. You can use also commanders like Minamoto, G.Khan but. So basically anything that that will do an insane amount of nuke damage.