A good idea for testing a class is against itself, usually with two wildly different builds.
1) Can the class kill itself in one round? If yes, consider lowering offense or improving defense.
2) Can the class survive against itself for more than 10 rounds? if yes, consider lowering defense or improving offense.
So, ability scores of 16/16/12/10/10/10, or 30 point-buy, which by lvl 20 becomes 28/30/18/10/16/10. She's wearing +5 mithral celestial plate, ring of protection +5, ring of natural armor +5, and using a +5 greatbow of speed, for which she has improved critical, greater focus, greater specialization and improved rapid shot. Her AC is 44. Her attack bonus is +37/+37/+37/+32/+27/+22. Her HP are 200.
If she's a slayer, her attack will deal 1d12+38 on a hit and 10 on a miss. From her attacks, one is naturally a critical, three are normal hits and two will miss, on average. However, being a slayer, she rerolls her second lowest roll and gets a better result, making another hit, turns her last miss into a critical threat and turns one of her normal attacks into a critical threat. Thus, on average, three critical threats (two of which confirm) and three normal hits, on average, against her own AC. That's a 300+ on damage, which means she can kill herself.
A knight with the same stats would probably use a +5 heavy shield and a longsword. She has an AC of 53 and, if she manages to mark her target after the charge she does the first round, the Slayer's attack bonus drops to +32. However, he still gets to do 60 damage/round despite the misses so he still wins in 3-4 rounds.
Essentially, against itself this class tends towards rocket tag if both are slayers, and the slayer wins over the knight.