Wraith is for a spell caster sovereign/hero focused empire. Make sure you have either fire or death, or both. Water/Death helps of course too (if you've played Resoln you'd know for sure). Have your champions blow mana and deal plenty of initial damage to take out key targets, or weaken the enemy with spells like mass curse. Then have your trained units finish off the targets to replenish mana and health. Pretty good when they have ranged staffs.
Having lower HP custom wraith races should focus on either ranged attacks or high armour values. High armour values mean you take less damage making that 3 HP gain effectively 'more' HP. EG if 10 damage was reduced down to 3 damage due to armour, then your hp gain from kills would be 'worth' ten HP. By this definition having Earth could help too (Cloak of Nature+Stone skin, mana upkeep replenished by Rite), but I don't think its as effective as avoiding taking damage by killing the amount of men in each troop before damage is exchanged.
You can work around the -1hp/level weakness, the question is why would you? Thing is normal trained units, for all races, are on shaky ground already - they're generally outshined by champions and whatever special units your faction/sovereign/race get (binding demons, henchmen, juggernauts, tamed animals, spiders, etc.). Most games I only use trained units if I get a bonus to them, say Gilden's excellent +1hp/level and armor bonus, or Magnar's fast and cheap slaves; without something like that, it's debatable if unmodified trained units are worth bothering with at all.
For a race that not only gets no meaningful bonus to its trained units, but actually gets the crippling -1hp/level (not to mention lack of armor), why would you waste time training them at all? Why would you look for ways to get around that weakness? There are better things to focus your production/gold/research on - champions, demons, spiders, assuming you're Resoln (no custom faction would choose wraiths). Resoln's racial weakness isn't an extra challenge to overcome, it's a reason to forget about trained units and never look back, it's that bad.