@Reddawn:
The goal when casting creatures with a spawnpoint is to summon what you need; if your opponent doesn't have much armor (or any) or bigger armored creatures, you don't need large creatures with special abilities; lots of little creatures actually give you more dice than one or two big ones, and are usually not very profitable for your opponent to kill or control outside of zone-targetting cards, or cards that affect the entire arena. To make cards like zone-attacks less attractive for your opponent, avoid clumping most of your creatures into a single zone; stagger them in different zones instead, if possible. Doing this also ensures that if an opponent attempts to block their movement with a Wall, only a couple of your creatures will lose actions getting around it, not your entire army.
On the other hand, if you see lots of armor and/or big creatures on the other side of the arena, you need to match them with strong, higher-level creatures of your own and/or armor, a defense, or other protection cards like Divine Protection, control cards like Agony to punish high dice or special attacks, and Regrowth on your mage to survive through multiple fights.
In general I agree that smaller creatures have better return on mana in terms of damage per turn (DPT). However, there are a few creatures that also have excellent returns in conditional circumstances:
Whirling Spirit - 4 + 50% push = +1.5 near a wall
Goran, Werewolf Pet - 5 + 3 vs. non-moving, injured target when Warlock present
Grizzly Bear - 7 + 1 piercing vs. non-moving
Darkfenne Hydra - 3 + 3 + 3 vs. non-moving
Corrosive Ooze - 5 damage + 1 corrode vs. non-moving
Iron Golem - 6 damage vs. non-moving
Sir Corazin - 4 + 4 vs. non-moving
So, it might be more accurate to say that large creatures are better when you can control your opponent's position? (At least until they make a large creature with a 1:2 damage to cost ratio that is not limited in this way.)
When it comes to small creatures I think the best two are:
Thunderift Falcon - 3 dice + flying, fast
Blue Gremlin - 3 dice + 1 piercing, defense of 8+, 1 armor, ability to gain fast & teleport
In part they are summonable with a spawnpoint that is not conditional, so you can spam at least one a turn. Additionally they are very hard to kill for their cost, which lets them fight much bigger creatures without dying quickly. The other big advantage is their mobility: with fast and either teleport or flying they can get around the board and apply damage where it is needed. Walls aren't effective either, which is a big plus.
I think when an opponent armors up to protect themselves from a swarm of smaller creatures it is reasonable to Dissolve or Acid Ball the armor to keep them exposed to the creatures. When they summon a big creature, it is also possible to swarm the thing to death with small creatures.
