Unlike Extermination, I actually maintained a very good win/loss ratio in Creatures vs. Soldiers. This gametype is basically a standard team deathmatch. Two teams battle it out for ten minutes and the one with the most kills gets the win. Ties do not count. At the halfway point teams switch sides.
The first thing you should do is familiarize yourself with the maps. Singularity only shipped with five or six maps, and they're not hard to learn. The action takes place at the same spots for the most part. Develop some strategies for these areas.
If you have any friends with the game, try to team up together. Be warned; this game has some issues with the multiplayer. It's hard enough to get two people in the same match, let alone on the same team. If you manage to do it though, be sure to communicate. Not many people use the game chat here.
Now on to the combat. On the creatures side choose a character you're comfortable with. The Radion and Zek are probably the most useful. Their attacks are very useful for taking out enemies quickly and building high kill streaks. Reverts are also good if you're skilled with them. Their healing puke doesn't seem to be as effective as the Healer's. Please do not use the tick though! Ticks have minimal health and their attacks aren't very useful. Playing as the tick is basically giving the other team free kills.
As a soldier I highly recommend picking the Healer. Seriously. Almost nobody uses him, especially in this gametype. His healing power will prove invaluable to your team's survival though. Hell, you barely even have to shoot. Just hang a little behind your team and heal anyone whose health is lowering. With you and your team constantly being healed, the creatures won't be banking neraly as many kills off you.
Pick a weapon you're comfortable with too. The Volk S4 shotgun and AR9 Valkyrie assault rifle are the most versatile. The Kasimov sniper rifle is not very useful however. The combat in this game is mainly close quartered and fast paced. Plus nobody snipes or camps.
Your perk selection should be based on the gametype. For the creatures, pick a perk that enhances the attack you'll be using the most. Such as more splash damage for the Radion, or increased melee damage for the Zek. I recommend faster recharge as your second perk. This replenishes your TMD energy quicker and allows you to attack more frequently. As a soldier I recommend the increased damage perk. This will put down enemies faster. The perks that improve your sprint and speed aren't very useful here as you won't be doing much running.
With your play style down, just use common sense. Don't go running into fights where you'll clearly be overwhelmed. If you see a teammate struggling, help him out. He might look at it as you "stealing a kill" but it's one less point for the opposition. If you're matched up with a consistently good team, stick with it. The teams sometimes switch in between games. If you just got a couple wins and are now getting beat, don't quit out. You could very well be put on the winning team next game.
Your Creature vs. Soldier wins, as well as progress for all your multiplayer achievements, can be viewed by selecting review from the main menu, statistics, then public match.