As there does not seem to be a TFTD strategy guide on this Ufopaedia web site, here is a starting point, which I will call:

Spike's Tentative TFTD Tactics

Initial Actions

Starting Base Location

The Straits of Gibraltar is not a bad location, covering most of the Med and the part of the eastern Atlantic. Or somewhere in the central or eastern Mediterranean might be better, covering Europe, Middle East and North Africa. (Not really sure about this!)

As soon as you have selected a base location, go to the Base screen immediately, to stop the game clock.

Facilities

Buy the following immediately:

Wide Array Sonar ($800K) - crucial to increase intercept opportunities
Alien Containment ($400K) - crucial so as not to waste research opportunities
General Stores ($150K) - space is immediately at a premium
Living Quarters ($400K) - to be able to fully utilise your existing Labs and Workshop.

If you are using the standard starting base layout, which is defensively very poor, build all your new base facilities below the row that contains your airlock. Be sure to build the General Stores and new Living Quarters in the gap between the two existing Sub Pens, as they will be completed first, thus making 2 more building spaces available. The new living quarters will help you dismantle the initial living quarters to isolate the airlock and sub pens. As soon as possible, build 2 new sub pens ($200K each) in the top left and top right parts of your base so that you can dismantle the vulnerable sub pens in the lower left and lower right.

Total cost $1.75M ($2.15M with new Sub Pens)

If you are using an "improved starting base layout" from XComUtil or another mod, you don't need to worry about rearranging your base. But you should still avoid building next to airlocks or sub pens.

Aquanauts

Recruit 7 extra aquanauts to bring your total to 15. Total cost $280K.

Sell all infantry items (Dye Grenades, Dart Guns, Torpedo Launchers, Torpedos) apart from:

The 2 Gas Cannons (GCs) and all ammo.
The 13 Magna-Blast Grenades
The 1 Hydrojet Cannon. Sell half the HjC-AP ammo (3 clips).
Keep 5 Jet Harpoons to arm your initial aquanauts until the additional GCs arrive. Sell 3 weapons, and half of the clips (10).

This sale yields about $18,600.

You will be using GC-HE in place of Torpedoes. You might want to hang on to the Torpedo Launcher and torpedoes until the extra GCs and GC-HE clips arrive.

Buy:

6 additional Gas Cannons to arm all 8 initial Aquanauts
Preferably a further 7 Gas Cannons to arm all 15 Aquanauts (total 13 GCs)
GC-AP and GC-HE ammo (in 1:1 or 2:1 ratio, depending on your tactics)
Additional Grenades (at least 12 - these are a crucial starting weapon to counteract enemy terrain advantages)
2-4 clips of HjC-P (for illumination)
Between 5-15 Chemical-flares, for illuminating areas not reachable by HjC-P
Thermal Tazers, for at least half and preferably for all 15 of your Aquanauts (this is essential for alien captures to advance research, and also as an initial counter-measure to alien Molecular Control)
2-4 or so Magna Pack Explosives in case you face strong foes early. They will soon be replaced with Sonic Pulsers.
Particle Disturbance Grenades if they are part of your tactics

13 GCs, 9 GC-AP, 14 GC-HE, 12 grenades, 4 HjC-P and 15 chemical flares costs $100K.
15 Tazers and 4 Magna Packs costs an additional $25K

Submersibles

Disarm the Craft Gas Cannon from both Barracuda. Sell all 4 cannon and all ammunition. This sale yields about $100K.
Arm the first Barracuda with Ajax + DUP, arm the second with dual Ajax. Remember which is which!
The Ajax+DUP re-arming operation will take 4 hours. The dual Ajax re-arming operation will take 7 hours. Consider doing the Ajax+DUP first, then the dual Ajax, to maintain an available interception capability.
Sell about half of your Ajax torpedoes, keep maybe 12 spares out of 25. This yields about $30K.
Buy 3 DUP launchers ($51K) first, and (only) then use your remaining money and space on as many torpedoes as you can get. Space is often the limiting factor.
When the DUP launchers arrive, replace all remaining Ajax torps and launchers and sell them. This upgrade will take 4 hours. Again, consider staggering the upgrades to maintain intercept cover.

Research

Set your initial research to Particle Disturbance Sensor. This is mainly to make a little money later on, and to provide your Scientists and Technicians with something to do while they wait for more worthwhile projects.

Research Sequence

Start work on Particle Disturbance Sensor as noted above
As soon as you possess a Sonic Pulser, switch to that (finish researching the PDS if you like as it gives something for your Technicians to do). The Sonic Pulser is a massive equaliser, quick to research, and large numbers are available to loot from the battlefield.
Once the Sonic Pulser is completed, start the sonic weapon sequence with the aim of getting the Sonic Oscillator craft weapon as quickly as possible. Skip steps wherever you can. The Sonic Oscillator is essential to make progress in the game, as the alien craft are too hard to kill without it. With fewer kills you get fewer missions, limited progress, and a future that is as bleak as it is brief. Fighting missions with Gas Cannon and no armour is better than having no missions to fight at all!
Once you have the Oscillator, research Molecular Control for M.C. Labs. (and make sure you have captured the necessary aliens by this time). This is more important to Battlescape combat success than researching melee weapons. Without it, the quickly increasing power of alien MC will take your teams apart.
Your next priority is Transmission Resolver. Intercepts are critical, they are what enable you to progress in the game.

The general approach recommended here is perhaps the opposite of the intuitive, instinctive approach. The instinctive approach is to research tactical equalisers such as more powerful personal weapons and armour for your Aquanauts. The smarter approach is to research more strategic equalisers - sub weapons, USO detection, and MC defences. It's hard to win battles with basic weapons, but it's impossible to win battles without successful interceptions.

Intercept Tactics

Watch the graphs for clues as to alien activity
Send out a Barracuda, slightly outside your sonar radius, as a picket ship. Every extra intercept is a huge bonus.
Keep your torpedo inventory light, but reorder torpedos immediately after every engagement, success or failure.

Battlescape Tactics

Avoid direct fire firefights wherever possible, unless you definitely have the upper hand. Use grenades and stay out of line of sight.
Identify your best grenade throwers (accuracy and strength/range), particularly those with high TUs who will get forward.
Identify grenade mules with high strength, who can carry a lot of grenades and relay them a long distance.
Give everyone a GC with at least one clip. Even a weakling can carry a GC preloaded with AP, if he carries nothing else.
The Gas Cannon is your primary direct fire weapon for all situations. Use GC-HE at long range, GC-AP at shorter range.
Do NOT bunch up under any circumstances: aliens use grenades freely.
Distribute as many Tazers as your encumbrance limits allow, to deal with MC incidents and to exploit capture opportunities.
Once Sonic Pulsers are available, use them in place of grenades, though initially only for tough targets (or multiple targets), until they are in plentiful supply.
When you see your first Calcinite or Deep One, capture it at almost any cost (as long as your Alien Containment is built!!!!)
Don't be afraid to fall back and abort if a battle is going badly, particularly if you are falling apart under M.C. The Triton is worth more than the Aquanauts in it, early on.
Even when direct fire Sonic weapons are available for use, be cautious in arming your Aquanauts with them. They are very expensive to own and operate. That money (from selling the weapons rather than using them) could well be better spent on base facilities, additional bases, or research (Scientists).

M.C. Tactics

As you will go some time without having M.C. Labs to screen your Aquanauts' MC strength, you must take very careful note of anyone who succumbs to MC, or even who is targeted. Remember that the weakest MC Aquanauts are targeted first, and if the enemy see one of your Aquanauts, they see all of them. Keep a ranked list of the weakest Aquanauts. If anyone succumbs to MC, stun them, disarm them, keep them out of combat, and dismiss them when they return to base. Be careful when deploying on missions with Aquanauts who have not been exposed to MC before - keep careful notes of this as well. For Aquanauts who have not been exposed to MC before, consider arming them only with Thermal Tazers until they are MC-proven.

Obviously if you get a really bad progress value when starting Research, you might want to change to a different topic.

TFTD Strategy Guide

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