SMITE

As Mulan, win a match with all weapons fully mastered.

Mulan has a new mechanic where she can level up her abilities the more she uses them, as well as being able to add a point to them at each level as normal. This achievements is for leveling up all her abilities using this new mechanic (not by putting five points into them).

As mentioned above, you level up ability masteries by using them, so the more you use an ability (your main three abilities) the quicker they level up. Each ability has three mastery level, Skilled, Adept and Mastered. Hitting enemies (including minions) with you first ability, Cross Strike, will increase the sword mastery. Hitting your second ability, Spear Thrust, will increase you spear mastery and with your third ability, Grapple, you'll increase your bow mastery. Using your ultimate, which involves hitting an enemy with each type of weapon Mulan has, will add to the mastery of the weapon she hits with,

You can keep track of this using the passive icon in game. You'll see a symbol with a hollow-looking sword, spear and arrow. As you use an ability, these will slowly fill. Once each icon has completely filled three times in a single game, you have to win the match to pop this.

I've seen a few comment related to this last bit, you must WIN the match in order to pop the achievement, not just max all your abilities.

This is most likely best attempted in Arena, as you'll be hitting lots of enemies in a small area, although Conquest is also a decent option as you'll have a lot longer to earn mastery levels.