first, some overall strengths and weaknesses.
The universal 4++ is arguably their greatest advantage: every model is free from relying on cover, which is excellent for a shooting-based force.
Most of their shooting, while pretty good, is short-ranged, and this is bad when they can't be in close combat. They need some sort of counter-assault unit if they don't want to be destroyed third or fourth turn.
The abundance of Bolt of Tzeentch means they will have no problem cracking open rhinos and other low-AV vehicles. Of course, they then need to deal with the infantry inside.
Now let's look at the units, and if they solve the aforementioned problems sufficiently.
Lord of Change
re-rolling 4++ is good, and he can hang back with nearly any unit and help any situation, many times all at once: Daemonic Gaze and Breath can be used on infantry, and at the same time, he can lob a Bolt at something else, and BS5 doesn't hurt either.
He's useful, but remember, he's expensive. Make sure he's worth it.
Chariots of Tzeentch, with Bolt, WaL, and MoS
No, there are no other kinds of Heralds of Tzeentch. These ones are the best 110 pts you will ever spend. The reason I don't bring Kairos is because he's more expensive than 3 of these, and these put out way more firepower. They're fast, so they can get into side arcs, and durable: 4 of them on the field can mess up opponent's target priority pretty well. Spread them out, and kill some for me!
Flamers of Tzeentch
These are your psuedo-close combat units: bring 3 4-6 squads with Bolt of Tzeentch, and let them do counter-assaulting, tank-wrecking, or aggressive anti-infantry as the need arises. Without an actual assault unit to back them up, you have to pick your fights more carefully than before, but they can still be very effective. Make sure to kill entire units at once, so they can't assault back.
First off, get the changeling, and write on your hand "whenever he declares shooting, remember the Changeling." I've forgotten to use him many times, get your 5 pts worth!
As for the pink horrors themselves, they'll be doing the bulk of the anti-infantry. Give them Bolt so they can threaten a variety of units. If you need a sacrificial unit, or something to take the charge instead of flamers, use these. They're versatile, use them creatively. Kill a couple models while holding an objective, or just tie up that one devastator squad. Get 5-man squads with Bolt, and they'll do just fine.
Get a squad or two of these to kill anything that Bolt can only tickle- they will, most likely, be suicide units, but try to get multiple uses out of them if you can.
Ok, so they're technically undivided, but meh. paint them Tzeentchy.
Use them for real counter-assault, but only use them to kill expensive models: a ten-man squad of termagaunts will keep it there the whole game, but it will rip through some terminators. Use Phelgm and Tongue to kill anything that you really need dead, from a heavy weapons team to a land raider.
If you bring a Soul Grinder, use the rest of your army to kill any melta your opponent has immediately. After that, he will be unstoppable! Especially if you bring 2 or 3!
Tzeentch Daemon Prince
Another BS5 Bolt is always appreciated, and he's no slouch in combat, either. If you give him Gaze or Breath, he may help with anti-infantry, but it's not very point-efficient. Just get another Flamer instead.
Keep him on the front lines, adding another Bolt or digging some flamers out of combat.
(Come to think of it, if Breath of Chaos is 30 pts everywhere you try to get it, does that mean that a flamer without it would cost only 5 pts? I want some of those, as Troops, preferably.)
Next post, I'll do a list as an example.