Basic combat hints
Terms like "perfect equipment" or "guaranteed tactics" are unknown in Perpetuum. The best warriors always prepare themselves for the situation at hand.
The most basic advice an Agent has to take is never to give up. If you suffer defeat in a battle, try to learn from your mistakes. Get to know what beat you, and maybe next time you can succeed with the same combination.
Your overall firepower is a sum of many different factors. Probably the most important part of character-based abilities are your extensions. A large part of the available extensions are dedicated to improving your combat stats. Choose extension from the main categories of electronics, engineering, and of course weapon usage.
Your enemy's targeting range exceeds yours? Install sharpshooting, or propellant mixing. Do you want stronger lasers? Install basic, or advanced optics. There are extensions for almost every tool and passive ability.
Extension Points - used to purchase extensions - are generated gradually. If you feel you are an underdog, be patient and wait for the required EP amount to accumulate.
Picking perfect tools
Use the right tool for the job, and focus on the task you want to accomplish: for a safe transportation assignment it's completely unnecessary to bring a combat mech (eg. Kain) as it has a low top speed and small cargo; bring a Sequer instead. Since it's a safe transport, time is money: put on a lightweight frame to increase speed.
Don't forget to check your robot's bonuses when choosing equipment. Minor boosts to firepower or electronic warfare modules - if only a few extra percent - can decide the battle.
Advantages and disadvantages
You can't have one without the other. For instance a huge mech (eg. a Tyrannos) might have a larger firepower provided by its weapon slots, but if your enemy is driving a small robot (for example an Intakt) he will have no difficulty leaving you behind as it has higher top speed.
The same is true for equipment. Buying an expensive weapon doesn't necessarily mean that it is superior to your previous one - it might have a larger damage factor, but will use more accumulator potential causing your robot's energy resources to run out faster.
Don't forget the relation between an object's hit size and the weapons' hit dispersion. The larger dispersion your weapon has (usually large weapons do), the harder it is for you to hit your target. This effect works the other way around too, you will more likely hit a large robot with a smaller weapon. A weapon in David's hand to defeat the huge Goliath.
Try to adjust the set of tools to the task you're about to accomplish instead of being versatile or universal; specialization beats generalization in almost every situation.
The first and foremost rule of engagement is to know your designated enemy. Rushing into a squad of unknown robots is probably the fastest way to get your own robot destroyed.
Try to measure your enemies first. How far they can target? What kind of damage type does their chassis tolerate the least? You've prepared for something else? Head back to the terminal and refit your robot, load another ammo type, and come back more suited.
Upon approaching the enemy the most common maneuver is referred to as kiting. Pick a robot from the bunch, lock it as primary target and attack it. When it tries to return fire and locks you, retreat, so it will lose you as target. (Obviously, you'll need to have a superior locking and firing range than your opponent for this to work!) If you did well, you won't be hit at all.
Natural elements (plants, rocks) or artificial objects (buildings) offer excellent cover. Try to observe your enemy's firing cycle and hide behind the obstacle when it's about to fire, then strafe out to shoot. Be careful though, as natural objects can't tolerate infinite damage - they'll fall apart eventually.
(Note that the above advice is more generally suited to NPCs than humans, since NPCs tend to be far more predictable.)
Sometimes your firing range will not exceed your enemies', making you a sitting duck, but there are certain ways to solve this problem. You will be under constant and heavy fire, so you need to reduce the incoming damage. Equip an armor hardener to deflect certain damage types, or use armor repairer to regain some HP.
Relying on your mates
There are many advantages that you might find useful when you're working together in a squad, mainly because of the skills provided by the NEXUS module system. It is also very useful to issue certain roles among squad members. In a combat-oriented squad, for example, an Agent who is highly qualified in repairing fellow members' armor damage can be an excellent asset for keeping the team on the field as long as possible. The squad is about to run out of ammunition? Maintain a supply route between the line of engagement and your ammunition storage bay.