system
Given the following post, respond as if you're commenting on Reddit:
human
[Attack on Titan] Congrats! As Vice Commander of the Southern Division of the Garrison Corps, you’ve been tasked with coming up with a plan to stall the Titans’ advance into Trost District and keep the civilians safe until they can all be evacuated. Good luck!

Commander Pixis is currently very ill and won’t be fit to lead the troops in defending the retreating civilians and slowing down the Titan’s advance. 

As Vice Commander of the Southern Division, you’re now tasked with this monumental responsibility. Fail and you’ll be demoted to a lowly common soldier. What’s your plan, sir/ma’am? 


Personally for me, before the Colossal Titan even showed up, I’d order for some of the land outside of Trost to become a heavily militarized no-man’s land. I’d also order for the small strip of land bordering the outer gate within Trost itself to be turned into an authorized military engagement zone where artillery fire and explosives can and will be used. More of those wooden defensive barriers shown in the anime will be made, with many standing by. 

Assuming that the Titans are arriving fast, artillery units at the breached gate will fire for as long as possible in order to delay the Titans and for there to be an actual military presence within the city, as well as the evacuation of civilians. The aforementioned wooden defensive structures will be moved strategically in order to block roads and alleyways which Titans may utilize. The plan isn’t to stop the Titans, but to merely slow and contain them. If the defensive barriers are doing their job, they should slow the Titans down enough for reinforcements to arrive, keeping them near the gate. 
Advance combat teams will also go over Wall Rose to engage the Titans if need be, however, they will only engage in combat if Titans are actually at the gate. 
If the Titans have made it through the gate and hugging the defensive barriers, troops will have to deploy ASAP. 

In my spin to the original Colossal Titan breach plan, I will order for troops to amass right at the vanguard in order to pack as much of a resistance against the Titans as possible. Meanwhile, assuming that civilians are still being evacuated, cavalry units will be deployed via lift/pulley system off of the wall and be used to distract and harass the Titans outside of Wall Rose for as long as possible. 
While the majority of Garrison soldiers will be at the vanguard, the Trainees (under Garrison command and supervision) will split into 2 teams and stand by by the flank, allowing for an orderly and tactical retreat of the vanguard (personally, I thought it was stupid for Trainees to be responsible for the Middle Guard since they just graduated training literally days ago. They weren’t experienced enough and should’ve had reserve Garrison troops lead/monitor them instead of having them be independent of any official military command structure. In the real world, you’re not going to send new Marines who graduated from boot camp less than 72 hours ago onto an actual combat deployment. Then again, Marines don’t have a 2 year long boot camp geared towards fighting giant naked creeps). 

Anyway, speaking of a retreat, civilians should be evacuated within 2 hours, and assuming that things work out perfectly with the civilians, the vanguard should slowly retreat into the middle while the rear guard comes in to help assist, forming another massive body of soldiers and Trainees to engage the Titans. Artillery units will also be permitted to use artillery within the city in the former vanguard area in order to pick off hordes of Titans, forcing the Titans to be attacked in the front and rear. If cavalry forces haven’t been wiped out, they’ll be used to enter the city utilizing a combined arms strategy with wall-mounted artillery units. 

If civilians are still evacuating after 2 hours, squads of elite Garrison soldiers/Trainees in the rear guard should be deployed as floaters into the city. If things got *really* bad, a controlled and orderly retreat should be conducted from the vanguard and into different sections of the city, forcing Titans to split and move in clusters. 
Speaking of civilians, in order to get civilians out of there ASAP, the pilot system used for the cavalry units should also be used to bring people across the Wall. 

Logistics and supplies should be spread out throughout Trost in multiple fortifications with personnel manning them at all times. They should have artillery of their own and should be fortified enough to hold off a Titan siege. Trainees will be responsible for guarding and assisting these logistics/supply depot/forts, as they are extremely vital to the battle.
gpt
First: whatever nut jobs thought it would be a good idea to assault the titans in hand to hand combat while flying around like a drunk spiderman, should be fired. Their reckless attitude will be a detriment to unit cohesion.

Second: Guns, lots of guns. Big bore is preferred.

Third: If the titans get passed that, wall of pikemen. Research tells us that titans have the same mass as normal humans, it's just blown up larger. Thus they have low density.  A few dozen pikemen in close formation with long pikes should have no problem skewering a titan, dragging it to the ground, and chopping him up like sushi. 

Finally: For specialty titans, Molotov cocktails. Is the titan armored? Large rounds coated in a steel jacket fired at super sonic speeds. If it can punch through iron plate, it can punch through a titan. Or at the very least knock it on its ass, where you can proceed to pour burning oils all over it.