Long story short, the spawn system in Last Stand is always the same, you'll always find the same enemies in the same waves, the only variation will be the starting gates.

The good thing is that you can tell where a particular enemy is going to spawn by looking at the previous waves. This goes not only for Colosseum and wave 9 banshees, but also for Anvil, where you can tell how the waves from 13 on are organized.

If you want to know what you're going to face, then the following links will surely answer.

The Bloodied Colosseum[dow.wikia.com]
The Anvil of Khorne[dow.wikia.com]

I haven't played here in a while, so bear with me if I can't remember much. As soon as I'll go back there I'll write down something more.

Waves to observe:
#1: look at the gates which remained closed.
#2: in one of the gates which remained closed in wave 1 there will be only Hormagaunts spawning (melee Tyranid mob). That's the hot gate.
#3: from the hot gate eldar warriors will spawn right away, while from the others they take more time.
#4: one gate opens last, from there a Force Commander and 2 Apotecaries spawn. Very useful if you're a cloning Sorcerer. This gate is always different from the hot gate.
#9: banshees will spawn from the hot gate
#16: clones from the hot gate, as soon as the wave before ends, remember this.
#17: the Lictor Alpha and a Carnifex spawn from the hot gate.
#20: every member of the party is teleported in a different gate, except for sorcerers with a doppelganger casted (but usually even without that), then the chaos lord spawns, revives the clones from wave 16 which then try to revive the bosses from waves 17-19. Shortly after, bloodletters start spawning in every gate. The clones from wave 16 are the only enemies which can revive, so killing them quickly should make the wave much easier.

Be careful if you're playing as a sorcerer and try to move away from the statue near the upper-left gate, since you'll probably be killed either by the clones or by the chaos lord if you're too close when the cinematic ends. The same happens during wave 20 in Anvil.

No clones here, but there are still some recurring spawns that may help setting up a defense.

EDIT: added info about the "fat gate", thanks random_noob

Waves to observe:
#1: look at the heretics' spawns. In one of them, after the first squads spawn, a second squad will spawn. That's the hot gate. Try to remember the other one as well since it's where the Leman Russ tanks will spawn.
#4: In this wave pay attention to each gate. The one which spawns more stuff is the fat gate (see further). The one which spawns Raveners + Lictor will spawn the Seer council later on. The empty one is the hot gate. The one which spawns Raveners and shortly after a Lictor is the Land Raider gate.
#12: in one corner a Predator tank will spawn. This is the same gate of wave 2.
#13: in the hot gate the Swarm Lord will spawn.
#15: pay attention to the gates spawing sentinels, one of them is the hot gate. In both of them a Leman Russ spawns at the end of the wave.
#16: the Battlewagon spawns in the hot gate.
#17: the Avatar spawns in the hot gate.
#18: 2 Predators spawn in the hot gate. The Great Unclean One spawns in the fat gate
#19: only stormtroopers spawn in the hot gate. I usually camp here. A Leman Russ spawns here, the other one should spawn in the other hot gate. Check the gate which spawns the Baneblade for later.
#20: if you missed which one is the gate spawning the Land Raider, check what appears in the corner where you end up. If it spawns Tact. Marines and then nothing else it will spawn the Land Raider as well.

In wave 19 my advice would be camping in the hot gate, stormtroopers are more squishy than Ogryns and therefore are - slightly - less dangerous. They will spawn behind you but a hero with a jump ability/good AoE damage should be able to clear the area with not much damage done to the party.
If there is a sorcerer in the party, then he should either copy a bloodcrusher, fire prism or actually anything with hp regen in any situation, or a predator (chaos ones are usually a bit worse than the space marine version) if he has Daemonic Gift of Sustenance.

The guide is pretty much done, I still have to check some things but the important stuff is already there. Feel free to add anything you know about this.