For our situation, the Defender will have 12 squads of militia allowed to deploy on the map as Hidden Units.
Squads or platoons?
If platoons, you can make a hardpoint: pick a building with impressive CF (At least a Heavy Building, Hardened obviously preferred) and park all the SRM platoons in it. Let's assume you get 4 of those, 8 of the Motorized, then you can stack 2 on each Level of the building. 4 SRM, 4 Motorized, Hidden so the first thing that trips them loses its movement modifier, that's going to make a big difference.
If squads, their firepower is borderline irrelevant, and just spread them around for maximum LOS.
The solution to that problem is to drop the building, so the real ideal is a building that permits the infantry to withdraw a step in to a fresh one. Or with the Motorized platoons, they have enough to cross the street in to a new building, even go up a Level.
If you go mobile on that tactic, don't stack them too high, but go side-by-side instead.
The other 4 platoons can be spread through the city. Find tall buildings, with extensive LOS options, and far from the fight so they don't get accidentally found. Make sure 1-2 of the platoons has LOS on the street(s) the enemy is likely to use so you can IDF them every turn. After all, buildings block LOS, so you may have several streets without LOS options if you're not careful.
(Salamander, Archer, 2x Catapults)
Victor, Grasshopper, Marauder 5-D, and Axman)
Tough choice.
Opposition will likely be a 2-3 Clan Omnimechs with a roughly equal amount of Elemental support, probably configured for close in action.
If you learn that type of OmniMechs they are, my recommendation is: go with the IDF lance when the enemy is slow (so 4/6 or less, JJs OK) and the brawler lance if the enemy is fast.
With just a few targets to destroy, being able to fire LRMs without answer for several turns is great, and you can space your LRM Mechs out such that one is getting the attention, while the others still hose LRMs downrange at short or medium modifiers once LOS is established. Just make sure to make the most of your armor advantage by rotating Mechs, and by not overloading on LRM ammo. Assume you'll only want about 6-8 turns worth of ammo, or prepare to have a turn where you're dumping ammo. (Easy when you're standing still doing IDF, and you know the enemy can make LOS in turn 5: fire everything and dump ammo in turn 4. If the enemy can jump behind you, you've waited too long)
The biggest problem here is the Elementals, they actually change this extensively. They can rapidly move through the city, will likely be suboptimal IDF targets, and once they close, you need volume of fire to put them down. Note that buildings, when you're adjacent to them, you don't make a to-hit roll, AND no Cluster rolls, so you can likely drop a variety of buildings on them when they close, especially if you have a few turns to prep while the enemy is out of range. (The brawler lance will basically be standing around waiting, so they can do a lot of 'prep' damage. But 3 JJs in the city gives them a lot of buildings to pick from.
IDF for me is:
Gunnery +1 IDF + range + Target Movement.
I don't do it when the spotter movement and LOS issues matters, or when the spotter needs to be shooting, unless it's a throw-away shot.
So, my assumption is Gunnery 4 + 1 + 4 range + 2 target movement = 11's. Drop to 9's at Medium. Drop to 7s at short (not impossible in cities, but likely you'll have 1-2 Minimum Range penalties too).
Between the Elementals and the probability of the enemy being 5/8/5 or better, I think the brawler lance has it this time. Especially if the missile Mechs arent mint.
One final reminder: remember that you can easily (enough) move through buildings to either break LOS, or to improve your own cover for a few turns. Building collapse damage is based on CF at the start of the turn, so as they can't quite drop a building in a single salvo, you can even tolerate that damage sometimes.
Beware of basements though. I think they're lame rules, but if they're in play, dont try going through buildings unless their base CF could withstand the weight of your Mech.