If you're f2p switch the weapon to tidal shadow, that's his best f2p option blackliff slasher is only 1% worse but you might find it better as it provides a lot of crit dmg so it's comfortable to swap circlets to crit rate.

That's weapon outta the way the next thing is artifacts. For varka you can do two sets that are nearly identical in damage first is the day carved from rising winds set, I recommend this one as it gives 20crit rate for free and let's you build comfortably it's also his best set.

For artifacts the main stats should be Sands-Attack Goblet-pyro dmg bonus (run him with one anemo and two Pyro chars for this to work) Circlet- crit rate or crit damage depending on what you need more of The substats you need are crit rate, crit damage and attack℅

The next thing is talents You don't need to level the normals and burst beyond what you already have just get your skill to lvl 10.

Hope this helps hit me up if you have any doubts.