I've been using a sprite perk which gives me +2 HP, but I was wondering if certain perks would be better in the right level themes.
Would it be better to switch +2 Hp for...
Low normal defense increase
Low elemental defense increase (in gremlin and construct levels)
Low piercing defense increase (in beast and slime levels)
Low shadow defense increase (in fiend and undead levels)
Low fire resistance increase (in fire levels)
Low shock resistance increase (in shock levels)
Low freeze resistance increase (in freeze levels)
Low stun resistance increase
Low damage increase vs slimes (in slime levels)
Low damage increase vs gremlins (in gremlin levels)
Low damage increase vs constructs (in construct levels)
Low damage increase vs fiends (in fiend levels)
Low damage increase vs beasts (in beast levels)
Low damage increase vs undead (in undead levels)
And, if it's better to switch, should I use a damage increase, status resist, or defense increase? (For example, in a fiery fiend level, should I use shadow defense increase, fire resistance increase, or damage increase vs fiends?)
BTW, I'm currently Knight Elite. I completed all rank missions at that level except HoH.
It depends on your situation. Basically, get defense if you absolutely need it. Otherwise, get offense to help you kill monsters faster and more safely.
The damage defense perks probably do almost nothing. Unless someone shows up with detailed tests showing otherwise, I would recommend never using them.
The health perk can be valuable. But once you find a vitapod, it's probably bigger than your perk, and hence your perk is not being used well.
The status defenses can be even more valuable, especially once they are bigger (+3 instead of +1). You can use these to push you over immunity thresholds, for example. Shock, freeze, and fire protection are the most important ones usually.
If you don't need any defense urgently, then get damage bonus or (later) ASI, CTR, or MSI. I almost always use the MSI+2 perk.