The biomancer Vorel wants every living thing to achieve its greatest potential. He will ensure that your creatures grow to gargantuan proportions. All you have to do is evolve them a little yourself first.
Creatures with the evolve mechanic get a +1/+1 counter whenever a larger creature enters the battlefield under your control. You need to play for the long game so that your creatures have time to grow into threats. Attack opportunistically with flying creatures while your other creatures discourage your opponent from attacking. There will be plenty of time for aggression after you've crafted the perfect army.
Many of your cards have effects that look for +1/+1 counters, making every evolution matter. Mutant's Prey allows your evolved creatures to fight your opponent's creatures, and Bred for the Hunt lets you draw a card each time a creature you control with a +1/+1 counter deals damage to an opponent. Either half of the "split card" Give/Take would be a solid choice for a Simic deck, but this deck is well prepared to abuse the benefits of casting both halves with its fuse ability.
If you're looking for more cards for this deck, take a look at the Gatecrash set. Master Biomancer is a 2/4 creature that gives your
other creatures +1/+1 counters as they enter the battlefield! Another place to look for ideas are cards with the undying mechanic from Dark Ascension and Avacyn Restored sets. These creatures return to the battlefield with a +1/+1 counter when they die, ready to battle again.