This is usually not a problem.
Dark Elves:
Sorgall (Lizard Breeder): For every 3 levels of the hero, 1 Predator joins (and continues to join) the hero's army. The damage of the Predator's "Lizard Bite" ability increases from 50% to 110% + 0.5% * Hero Level.
Vaishan (Black Hand): For every 2 levels of the hero, starting from the first, Scouts in the hero's army receive +1 to defense and attack. For every 2 levels of the hero, 1 Scout joins (and continues to join) the hero's army.
Letos (Poisoner): Each enemy unit can enter battle already poisoned (under the "Plague" spell). The probability of this poisoning is 3% for each level of the hero.
Erin (Witch Coven): For every 3 levels of the hero, 1 Witch joins (and continues to join) the hero's army. Witches have a chance (equal to 10% + 2% for each level of the hero) to cast a spell on the hero's target.
Railag (Chosen of Chaos): For each level of the "Luck" skill, the hero receives twice the "floating" bonus to parameters (i.e., +2 instead of +1).
Shadia (Dark Mystic): The "Dark Ritual" ability is enhanced. It allows the hero to replenish mana above its maximum value: an additional 1 mana is given for each level of the hero.
Sinithar (Catalyst): The amount of mana consumed by enhanced spells is reduced by 3% for each level of the hero, rounded down.