Strategy answers the question "What do I do?".

Tactics answers the question "How do I do it?".
In other words, tactics is the way of carrying out strategy.

Abbreviations list:
AA - anti-aircraft unit or anti-aircraft defense structure.
AT - anti-tank unit.
TG - tactical group.
AI - artificial intelligence (the one which plays against you in Skirmish mode).

It is complex to single out one strategy for all cases and all generals in Contra. There is no such thing. Each general has a unique set of strategies and tactics suitable for different situations.

But first of all, one need understand a unique feature of Contra. It is the rank system. All units, buildings and upgrades are divided into 3 classes: early, middle and late technologies. Early technologies are available from the start. Middle technologies are unlocked automatically after you reach rank 3. Late technologies are unlocked after you reach rank 5.

Thus, experience becomes a critical resource. You must protect your units to prevent the enemy from gaining experience and ranks while you kill more enemies to rank up faster. The one who gains 3rd rank first has a strong advantage as a Rank 3 army is significantly more powerful than a Rank 1. Like wise, Rank 5 is a substantial advantage. The 5th rank gives access to super weapons and super units which can level bases alone.

Some common tactics and strategies are listed below. But it is not a full list. You can always think up strategies and tactics of your own.

Tips:

Tip 1. Don't spend your money to buy mines around buildings or building security systems without necessity.

Tip 2. Don't try to attack strong enemy forces with single units. Gather at least small group before attack.

Tip 3. Many new players like to use shapeless heaps of tanks or other units and don't care of losses. It is bad idea for Contra. Each lost unit gives experience to your enemy. Enemy gains next rank faster. Next rank unlocks new technologies for him and allows to produce more powerful units. Spare your units, use them carefuly, don't allow enemy to get experience.

Tip 4. If you need to protect your base from stealth units, there can be 2 ways: to build defense line suppied with scouts or to build mines around.

Tip 5. Defense structures are quite powerful in normal ZH but not in Contra. So, you need combat units to support your defenses. Defense structures are vulnerable to artillery. Hence, you should use helicopters, planes or another couner-artillery measures. Moreover, defenses don't detect all types of stealth units (the most of them detect stealth aircrafts only). So, you need to use Radar Vans, Listening Outposts or Sentry Drones.

Tip 6. Scouting is quite important. It helps determine which tactic should be used. Put scouts in front of your defenses. They will warn you about enemy attack in time. Move scouts in front of your attack force. They will warn you about danger. Don't forget that many scouts detect different types of stealth units. Detection capabilities are explained in unit descriptions. Read them attentively and use scouts properly.

Tip 7. When you play in a team against Stealth general, it can be useful to mark his key structures with beacons. Both you and your teammates will see these beacons. It will allow to coordinate your attack.