Hi, your mobility really depends on your opponent and the scenario being played, but there are some ways your army selection can make you more flexible.
If deep striking is allowed I generally field at least a team of XV8s and a drone squadron that I use to deep strike and hit any part of the enemy's line I want. This is useful for taking out enemy armour as crisis suits firing fusion blasters and rapid firing plasma rifles at the rear of leman russ's and their ilk have a easy time getting critical hits. I have even used a deep striking drone squadron to kill a leman russ from behind
The rest of your army works best if you set up part of it as a fire base, using fire warriors in good positions, broadsides and even a hammerhead to reinforce the line. I normally deploy a stealth unit in front of these to harass the enemy as he closes and even some kroot to murder any units that get too close (get a shaper and some hounds even).
You can then use a 'hook' of units to swing forward on either flank that might consist of a devilfish (or two) carrying some fire warriors forward to make life difficult for your opponent. This move can then be supported by a number of units, a hammerhead for one, some kroot or stealth suits deployed in advance or even crisis suits as they can easily keep pace with the advance (or deep strike to meet them).
These are just some basic ideas that I have already used to great effect, but of course you can adjust them for your own ends, like if you want to be really nasty deploy some stealth teams near the enemy line, and early on in the fight deep strike 2 teams of 3 crisis suits with drones, and 2 drone squadrons and proceed to slice and dice..
