Petra, huh? This is the first 'Rate the Unit' I have commented on, but here we go! (Also, brace for a wall of Text). DISCLAIMER: this is just from my point of view, while rating her in a vacuum.
Skill Levels: D+ and a Boon in Swords means she can get into Mercenary at base, which is something. E+ and a Boon in Axes and Bows means easy access to Death blow, Wrath, Accuracy +20, etc. In all honesty, I don’t think +20 Hits is that great, but hey, that’s just my opinion and has no impact here. D and a Boon in Flying, however, is very good, as with just a bit of Lance training get her to Darting Blow and latter - possibly – Wyvern Lord.
Abilities: Hunter’s Boon only has 1 use if you ask me: Monsters with 100+ HP, which is of variable use depending upon the route and difficulty. Battalion Wrath opens the door for Vantage+100% Crit setups, but Petra isn’t the only character to do this, nor is she the best at it. The 2 viable builds for her (in my humble opinion) is the afore mentioned Vantage + Crit setup or a Dodge-Tank build.
Arts: Waning Shot is of fair usefulness against some Bosses and Beasts. If you do try to use Wild Abandon or Diamond Axe, then I daresay Hit +20 is quite useful, but aside from that, her Axe arts are underwhelming at best. As for Sword Arts, Bane of Monsters is of questionable use while Finesse Blade can maybe do a lot of Damage, as Petra’s Dexterity is 7 Base +50% Growth.
Stats: 9+40% Strength is passable, 7+50% Dexterity is solid, 10+60% Speed is only surpassed by Yuri, but unless you bought the DLC, he’s not a consideration. 7+45% Luck is decent, but Luck isn’t what I’d call good in the best of situations.
Her other stats, however… 25+45% Hp is mediocre for a Front-line unit, 3+25% Mag is not worth considering, 5+30% Defense and 2+15% Resistance means if she does get hit, it WILL hurt. 6+35% Charm depends on the difficulty: Hard is irrelevant as enemies (Aside from actual Characters) are a joke when using Gambits, while in Maddening she’ll be target practice.
Classes: Beginner Classes- Soldier/Fighter for Reposition/Shove, or Myrmidon for the +2 Speed (It’s good in the Early Game!)
Intermediate classes- Brigand for Death Blow, +30% Hp, and +10% Strength, Pegasus Knight for Darting Blow and +10 Speed, or Archer for +20 Hit
Advanced- Assassin for the 6 Move and sprinting through Terrain, +20 Dexterity, and Speed, Warrior for Wrath and +15% Strength, Swordmaster for +10% Strength and +20% Speed, or Wyvern Rider for being a Flier and +10% Strength
Master- Either Wyvern Lord (Preferred) or Flacon Knight
All in All, Petra is good for either going down the Sword Line ending at Assassin (Or Falcon if you feel like getting her E rank Lance to B+), the Axe Line ending at Wyvern Lord, or - I recommend against this - the Bow Line ending at Bow Knight (Or Sniper for Hunter’s Volley and not having to get her Lance to C and Riding to A).
The Verdict
Petra is very Versatile, almost too much so. How good she is depends upon her build. Personally, I would have her in either Assassin or Wyvern Lord, using either Sword or Axe Prowess Lv5, Alert Stance+, Vantage, Wrath, and Battalion Wrath. In such a scenario, I’d give her a 8/10. She crits stuff to death – or failing that – dodges the retaliation.