New to GW and Sin: Where to start?

make sure your team has two monks that can split both sides and i would go with what zeAliX said as there is not shortage of bodies on that map

also as a sin on that level you want to wait and get the enemies that come in through the gate as not only are they the biggest threat to the tree singers/your monks but you'll have less of a chance of being the main target of the plagueborn

incase you don't kno....
mm = minion master

controls alotta minions that can deal damage and shield as a meat shield for the rest of the party