There is a guide on reddit and the various pages.
Basically, Lonqu at three star has Vantage 1 and 2 unlocked. Lon'Qu at 4 Star has Vantage 3 unlocked.
You're allowed to pass three skills at a time, if you were to inherit a three star LonQu Tharja could learn vantage 1 then vantage 2 FROM LON'QU, she won't be able to inherite Vantage 1. Then learn 2 and three on her own, every rank has to be inherited.
So if you want Vantage three on Tharja, you would raise lon'qu to four star then learn his skills from him then Vantage 1 > Vantage 2 > to Vantage 3.
You get SP by killing stuff that's up to ~five levels lower, so have her kill stuff in training grounds.
You need to upgrade a 3* Lon'qu to 4*, but you do not need to learn Vantage on him - units can transfer any skill they can learn at their star level. You can transfer 3 skills per inheritance session, but that would need to include all 3 levels of Vantage. The reason to upgrade Lon'qu is because he doesn't learn Vantage 3 until 4*, and you want to do this because he is actually the only way you can pass along Vantage 3 without sacrificing a 5*.
You can get more SP by killing enemies in eighth stratum, sometimes seventh stratum. I forget the exact level range, I think it's 6 or 8. But you get 3 per kill. It's not that hard on a caster like Tharja.
I dunno if I'd put Vantage on Tharja to begin with but Vantage is so good in general that I guess it'd work. I like the Darting Blow/Desperation synergy a lot.
OMG thanks guys! I knew you guys would actually know how to play. lol So, darting blow/desperation? Might check that out before I make any decisions. I'm going to have to do some more looking into different builds for Tharja.
Is there a limit to how many inherited skills I can put on a character? Like can I put Draw back, Fury 3, Deparation 3, Vantage 3, Renewal 3, Hone Speed 3, and Hone Attack 3 and just equip and unequip the skills as needed?