Just to help anyone out who's struggling with it, this is what I do, and it works for single player too, although I'm still learning. I'll likely update my plan over time, but this works for me at the moment
1) use the prep time to zip around the frontline area and find the dragonator/Wyvernfire switch, eyeball where the dragonator attack (huge spikes set in the walls etc) will come from so you know the killzone. Wyvernfire is an area attack that you target directly when you press the switch. Notice where the furnace is in the area. Using it reduces prep time for special weapons and turret repairs. You can't stoke it before the first wave, but you can in between waves. Do this. Mark where the counterattack gong is. Make a rough plan in your head of where these all are, Then start placing installations. You've got time.
2) put down bombs in the ground installations.
3) Ballistas- crew one of these to start with. Go for the head with the special ammo.
4) fill out any unused installations with auto turrets. Have a look to see where other hunters seem to want to crew something and work around them, but ideally crew the ones near the dragonator/wyvernfire/counter signal.
When you're ready, press the right stick to start the wave early if everyone is good to go.
When the first line of turrets are getting overwhelmed and the artillery isn't putting down enough fire to stop monsters getting past the halfway point between the frontline and the gate, sound the counter signal. The special weapons need good timing (and aim in the case of wyvernfire), which is why they need someone to babysit them and take advantage when they become available. Save them for the final boss, but timing is critical, particularly with dragonators. For ones in earlier areas it will run past them quickly and you need to be ready.
If dragonators have been used on the monsters, stoking the furnace is a good use of a couple of seconds if you are heading past it.
With the first wave down, you've likely had a ton of upgrades available. Swap ballistas to cannons and then machine guns, crew one of the latter yourself if you can as they are hella effective. Upgrade the ground installations to the village characters if they become available. The dragon bait is to use on the ground installations directly in front of dragonators- it's to gather large monsters there for enough time for you to hit the switch.
If you've picked up any monster drops that are bombs, place them somewhere where they'll be useful- next to bamboo bombs is good. You can drop your own traps and bombs too if you're feeling fancy.
When the counter signal is up, get in there and take stuff out on the ground, starting with the monster closest to the gate. However, if most of the monsters are flying and you have a machine gun turret in good nick, there's something to be said for one of you still providing covering fire. In single player, get down there and hunt.
When the chef becomes available, she's a good turret to install as it's an auto machine gun- get this up and running on the final boss.
Fugen acts as a sort-of smart bomb, he's a good one to save for the apex.
Apex monsters hit like a ton of bricks, and carting costs time. In multiplayer, don't be afraid to crew a turret if you can contribute more that way, but in single player you'll have to directly counter it yourself.
If the Apex crashes through into the final area, it's worth dashing around and setting up auto turrets quickly, using any wyvernfire that's available and then stoking the furnace (you might get a second use) particularly if there isn't a counter signal up. Do that, then bash the signal.
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