Chances are nobody has played this faction before, which plays to your advantage.
The 2 deceivers run side by side. Deploy them on the front line. You want them in combat from turn 1. Throw the one without the cloak in first so you get 10 attacks on 3’s, 2’s (with a chance of 2’s, 2’s) at rend -3 and d3 damage. They melt stormcast, eels and big characters with a 4+ or even a 3+ save. Everybody will be expecting the skitterleap so you will need to use up your opponents dispels to get it off more often than not. I tend to use it on the arch warlock. Throw him into combat against lone heroes or small units after using the once per game d3 damage shooting attack. With mystic shield he is on a 3+ re roll 1’s.
The Majority of your gutter runners should be set up off the board. A lot of opponents will give you first turn so you can’t double turn with deceivers, and the prospect of not knowing where 100 models will turn up generally puts your opponent at unease.
Chaos. Meh. Remember the unpredictably destruction rolls when piling in. The 6’s come up more than you’d think and +1 on these units turns them into blenders.
Artefacts, Command Traits & Abilities
The doppelganger cloak is great on the general, it keeps him alive longer and churning out damage. Mortal wounds will finish him quickly so try to pick off wizards or avoid them if you can.
Lord of war is probably the best trait now. On a 4+ you pick a unit within 3″ and give it +1 to hit. This is best used first turn on the general himself for 5 attacks on 2+ 2+ rend 3 d3 damage. It makes the shooting attack better too. Every little helps. After that, you want it on gutter runners. On 25mm bases you should be able to get full units of 20 in combat. Putting out 40 attacks on 2+ 3+ rend 1. With the command ability they can be re rolling to wound as well….
In the first turn you will have 3 command points. Save these for re rolling charges on the gutter runners that you bring up if you need them – which brings us on to cogs. Cogs are huge for this army. Have the grey seer cast them, so you can re roll failed attempts on a 2+. Suddenly your gutter runners are charging on 7″ with a re roll. You should get 2 or 3 units in first turn. It also means that even at 24″ apart, your deceivers are on a 8″ re rollable charge turn 1… Chances are if you deploy on the line then you will be closer than that anyway. Always keep 2 units of gutter runners back for objective play. Keep your seer near whichever unit you might need to use a command point on to make immune to battleshock.
Alternatively you can drop some units of gutter runners back and ally in a plague priest for an auto re roll to wound on 1 turn without expending a command point. Works great if you have blocks of 20 runners shooting 40 attacks at range. The 5+ re roll makes them much more effective.