Monstrous Creatures are by definition monsters and that stride across the Warhammer 40k battle field crushing everything before them. But they are not invulnerable, There are many tank busting weapons out there that will blow a big hole in your big guys.
Your Monstrous Creature will be a prize target in your army that will draw a lot of attention and in coming fire from your opponent. You can use this to your advantage by drawing fire away from smaller weaker specialist units that have a specific job to do ( Gain an objective, blow up a tank & etc ). However with this expected in coming fire you will want to place you monstrous creature in a location where it can move, but will have some cover that will keep it alive longer. Being big, there probably not be too many choices available. This in turn means that you may want to place your Monstrous Creature down first when you are setting up, so that you do not clutter up the best spots with smaller models that can be placed elsewhere or around the base of your Monstrous Creature.
All monstrous Creatures have a number of special rules. These are Fear, Hammer of Wrath, Move Through Cover, Relentless, and Smash.
Monstrous Creatures can shoot two weapons
This rule gives the monstrous Creatures the ability to become a walking gun platforms specially as many of the Monstrous Creatures have access to big guns, many of these are long range weapons too.
Now if you add in the Relentless Special Rule (p41) that allows you to move, fire heavy weapons, and assault on the same turn, you can see that you can get quite a bit of usability from your Monstrous Creatures each turn.
Monstrous Creature Movement
Monstrous Creatures get the Move Through Cover special rule. This can catch your opponant by surprise if you are lucky. Otherwise Monstrous Creatures move as normal. If you are moving up the table, keep an eye on the larger pieces of terrain that you can hide behind and gain some useful cover save from.
Monstrous Creatures automatically fear. This is useful if you are in close combat with an army with lower leadership scores like Orks and Imperial Guard. Not much to this one other than remembering that you have this special rule when you get in to close combat and factor in a possible higher kill rate when calculating which units you are going to use to assault which units.
Slow in combat
The Cause Fear special rule is important to a monstrous creature because they tend to be slow in combat. If you can slow the enemy down too, then you will have a chance to cause damage before your enemy has chance to strike you, or at least get the chance to strike at the same time. If you do not get the chance to slow down your opponent, here are some other options.
Assault smaller units
Monstrous Creatures usually have a high toughness value and a few wounds, this means that they will be able absorb an amount of punishment while it is waiting for it’s initiative round. However this ability has it’s limits so you want to minimize the risk where you can. Assaulting smaller units that do not have monster killing special weapons is a good example of this. Chose targets that you can beat where you can, and use an y heavy weaponry to thin out the ranks of larger units from a distance before you assault.
Get a Little help from your friends
Close assault always goes better if you assault a smaller force with a larger one. One way to do this is to team up with other units to assault a single target. This will mean that you have to focus a percentage of your forces to the one task, so make sure that removing this enemy unit is in line with your mission objectives. This will require the ability to step back mentally ( and physically if it helps), and have a look at the bigger picture before you dive in. Also it may take a turn or two to get the right units within striking range, so a bit of forethought may be needed too.
Flying Monstrous Creatures
Some armies have Flying Monstrous Creatures instead of aircraft. I tend to think that Aircraft work better on larger tables and larger pointed games, however Winged Monstrous Creatures can act as jump pack troops too. This means that they can deep strike and move quickly around the table.