Chapter 7: A Trap is Sprung

Objective: Reach Mako Storage
Go all the way to the SW. When you reach a dead end, Tifa will suggest a "creative" way to proceed. Note that you are generally traveling down here, and the location numbers getting larger (e.g., from B5 to B6). If you keep that in mind, you shouldn't have any trouble getting to the reactor. You're about to encounter a lot of human enemies, so this would be a good time to swap in Fire Materia for Lightning Materia on Barret's weapon.

Objective: Sentenced to Death
You will need a keycard, which you will find conveniently glowing behind you along the W wall. Use it to access the console and dispose of an M unit. This is where things can get confusing. Although the checkpoint marker is South of you, you actually want to head North. This is because the marker is on a different level. When in doubt, follow Tifa and Barret. Also observe that your team is now heading upwards. In the room on B7, you'll find a keycard along the West wall and a choice of 2 ways to undermine your upcoming enemy. Really, it's your choice here. I chose to disable the AI so that it would slow down. Don't overlook the free Lightning Materia in the NW corner. Go out the South door to B6. After beating the enemies in the passageway, look in the SW corner and find a bonus keycard! Continue up to the door that says B6. Clear the enemies and collect another keycard from the West wall. You have 3 choices but 2 keycards. Again, this is up to you. Exit North and see the purple Chest ahead. Purple chests mean weapons. In this case, you get a pair of Sonic Strikers for Tifa. Equip, set to auto-upgrade, fill with Materia, and make note of the new Focused Strike ability to learn.
Tifa's Sonic Strikers and the Focused Strike Ability: The Sonic Strikers provide about the same 3:2 ratio of Physical attack to Magic Attack as the Leather Gloves, though both numbers generally run somewhat lower for the Strikers. Upgrades give Tifa the second highest Speed boost (21) of any of her weapons and the largest MP increase (31) of any of her weapons. A late upgrade gives a substantial 50% chance of Concentration Activation, and a postgame upgrade will reduce MP spent on healing by 20%. The Focused Strike ability costs 1 ATB to use, and it is one of Tifa's most useful Weapon Abilities. When executed, Focused Strike has Tifa jump back to avoid being attacked before launching forward and then leaping back again. The attack does damage, but Focused Strike shines because it boosts Stagger impressively, especially when the enemy is Pressured. As a bonus, Tifa recovers a whopping 70% of an ATB bar just by executing this attack. Focused Strike combines nicely with Whirling Uppercut which, you will recall, restores at least 40% of an ATB bar. When faced with a Pressured enemy, Tifa can execute Focused Strike to increase Stagger and then follow up with Whirling Uppercut. Now she already has more than one full ATB bar (70% + 40% > 100%) and can repeat this sequence without having to pause and refill ATB. For many enemies, this will cause Stagger very quickly. The Proficiency Bonus is self-explanatory, but of course, that doesn't keep me from explaining it anyway. If Tifa's Focused Strike attack Staggers an enemy, then the Bonus is earned.
Continue uphill to B5. Defeat the enemies and find 2 keycards -- one along the East wall and one in the NW corner. Choose 2 of the 4 systems to flag for disposal and exit South. Follow the linear path to the marker at B4. Defeat the Cutter to open the way forward.

Objective: Storm the Gates
When you get to the Front Gate level. Unlock and enter the room and patiently try to coordinate pushing levers with the rest of the team. This will unlock the door you need to proceed, but before proceeding you want to also unlock the door to the Disposal Room. Go back to the levers and synchronize pushing the levers 4 times. You will receive the In Lockstep Trophy. This is unbelievably difficult and frustrating, so don't get discouraged if this takes dozens of attempts.
Enter the Disposal Room (it's the purple Discovery marker on your Map) and pull a switch. In there you will find a number of "chests" to open (for me it was 6 chests) and a rare orb of Magic Up Materia.
Loop around to the North to find a bench and a vending machine that will sell you Music Disc #2. While you're shopping, you should buy the 3 Megapotions that are offered at sale prices. They might come in handy in the upcoming battle.
When you're ready, exit West and then North to the marker to fight the Chapter's boss.