Turtle tactics are a last resort for players with no imagination, who enjoy winning far to much. This tactic is frowned upon by all civilizations other than Kraan.
Basically, minifigs are all given a long-range weapon, then they all hide in cover, and shoot whenever an enemy comes into range. This is a very low risk tactic, that should not ever be used as it makes games boring. Unless of course you are just suppressing the enemy so that other minifigs can outflank them.
if your opponent turtles, you have the right to impose an Unmighty Pennie of Doing Nothing for every turn on which they Do Nothing, and then taking the free time to mass your forces for a rapid anti-turtle rush. This will either overwhelm their defenses, or at least end in a spectacularly bloody fashion, thereby proving you were more ossum than your opponent.
A completely different variant is the "Turtle" phalanx formation, which allows a stationary group of shielded units to massively up their resistance to bows and light ranged weapons. This is cooler if only because you get to pretend to be a tank.
If your opponent regularly employs turtle tactics, there are several things you can do to prevent him from using them. First, you can simply play without accuracy modifiers for cover, as that removes any incentive for staying behind cover. If that doesn't work, there are several specialty units and creations you can construct to counter turtle tactics, such as Dropships which can deploy your troops behind enemy lines or extremely powerful Siege Weapons which can blast apart or burn the terrain the enemy is using for cover.