Gathering intelligence Edit
You are going to need to find entrance routes to the enemy base, preferably by both land and water, but just water will do (going by land kind of defeats the purpose of an amphibious assault, right?). Make sure that you have selected one that your units wont be mowed down by enemy fire. If you can assault their naval base, do so (remember that units with the Hover designation cannot be hit by enemy torpedoes).
Type of units to use Edit
For the direct assault units you usually have a very limited amount of units to choose from. All T2 factories have one T2 amphibious tank that can be used and there are a few others amongst the other factions. Naval fire support can be provided, which is described later
The UEF only has the Riptide as a decent amphibious assault unit early on. The Riptide is quite durable, but it is short ranged and must rotate to face its target to fire. 20 or so should be used minimum, and they need long range support from Battleships and Cruisers to be truly effective.
In the later stages the UEF gain the Percival. This unit is definitely worth considering. It, however, has no defense when in the water so you must use submarines to clear a path.
The Fatboy is the only amphibious experimental that the UEF has. Unless the enemy has very tough defense or lots of naval power, (or an experimental), the Fatboy can do a great job in taking out enemy defenses. Even its death can take out parts of their base. Usually combining a Fatboy with Percivals can make both far more effective than if either were on their own.
If you require air support use the Atlantis to make an airborne assault.
Like many of their units, the Cybran amphibious units are able to function individually quite well. It is still suggested that you mix units for best results.
The Wagner is the standard amphibious tank for the Cybrans. It should not be used on its own though. Its torpedoes leave much to be desired, and its weapons, while fairly powerful, are severely lacking in rate of fire. Small or lightly defended outposts are the best targets for these.
The T2 Destroyer: Salem Class is one of the best amphibious units. It has large amounts of health, a powerful weapon, and needs little or no support. Three or four of these landing next to your base can be quite a scare. Imagine having 20 of these show up. Salems are quite tough to build in large numbers, but if done right they can prove invaluable.
The Brick, one of the best T3 units to use for amphibious assaults. While its torpedoes are relatively weak its Heavy Disintegrator Pulse Laser is very powerful. Large numbers of these are great for taking out shields and coastal defenses.
The Monkeylord is the cheapest experimental one can use for amphibious assaults. Its Heavy Microwave Laser and AA power allows it to function on its own with ease. It is also relatively easy to get three of four of these once you can start building them. And the fact that they have stealth is a major bonus.
The Megalith is the biggest and most powerful Cybran amphibious unit. It can bombard the shore long before it beaches and can even stop and build support while still in the water: A major bonus. Using the Megalith with several other amphibious units can guarantee mass destruction within moments.
At T2 the Aeon gain the Blaze. The Blaze adds some much needed firepower to the T1 arsenal. In large numbers not even naval units stand a chance.
The T2 Mobile Shield Generator: Asylum is also added to the list of units to use. The Asylum provides much needed survivability to the land units. This allows the Aeon to attack more heavily defended positions.
The Aeon also can use their T2 AA unit the Ascendant to support their Amphibious assault forces.
Their are no real special land units that can be used in T3. The T3 Shield Disruptor: absolver adds some use to the Amphibious assaults, but other than that there is little to use.
The Seraphim are much like the Aeon in that they have more ground units to choose from.
The Serphim's first amphibious unit is the Zthuee. This mobile artillery is great for taking on enemy positions from the sea. However, they are worthless in direct confrontation.
The Seraphim also have a hovering AA unit called the Iashavoh. This unit is well suited for supporting the Yenzyne.
At Tech 3 the Seraphim gain the Othuum Siege Tank. The Othuum is one of the most powerful T3 units that can be used for Amphibious assault.
The Athanah is the mobile shield generator for the Seraphim. It is quite useful for supporting large numbers of hover units and ups their survivability.
At Tech 4 the Seraphim gain the Ythotha experimental Assault Bot. Defenseless while under water, once on land it can quickly tear apart a base with its massive firepower and AA turrets. Little support is needed for this unit to do its job.
Sea Borne Support Edit
Don't forget that you can support your amphibious assault with a Battleship or two, and UEF/Seraphim Cruisers with their long-range missiles and advanced AA if available. This can be useful for taking out places where your enemy is heavily entrenched. But remember: once you fire that barrage your enemy will be on the alert! If you can, have the shots land just as your units get within range of the enemy turrets. Then you can continue to bash any opposition while your Battleships help. If an enemy target has moved inland don't forget to re-acquire a target because your battleship could run into enemy torpedoes and ships. Keep those battleships moving: if they stop for too long your enemy will note their position and could send some artillery shells your way. Finally, remember to support your battleships with some smaller units to fend off attacks.