Elwin is the only hero that carries over in the campaign, so make sure he is
powerful, and make sure he learns as many spells as possible. Also, since
he'll get the summoning skill, it's best that you travel with a full army so
you can control which creatures he summons. Similarly, it's better for Elwin
to be a Warden or Summoner than a Beastmaster, although you'll still want to
pick up some combat skills.
The best Nature units are elves, griffins, and faerie dragons. Elves are the
only Nature ranged troop outside the creature portal, and so by default
they're better than white tigers. Griffins are a good utility troop since
they can fly, and the faerie dragon bonuses are better than the phoenix
bonuses. Phoenixes are better for summoning.
As for the creature portal, I like to use a combination of water elementals
and fire elementals since they compliment each other, and since the only
other Nature ranged unit is the level 2 elf. (Waspworts are too slow to use
in an army, but they're fine for castle defenses.)