TACTICAL SUGGESTIONS:

In general, when using powerful units, it is worth bearing in mind, that they will attract a lot of firepower. Experience has shown, that it is a wise tactic to always use more than one tank at a time. One tank is easy to destroy, but two are much harder. Remember that a single powerful weapon that is destroyed is a waste of points. It's reasonable to assume that when we take powerful units, it's because we want to use them. Of course the enemy is going to try their hardest to thwart your plans and you should assume (and therefor have a contingency plan or backup) that you will loose at least one powerful unit in the first turn.

Because of its long range, the Fire Prism can be kept a long way back from the front of your deployment zone. Also there is no need to upgrade the shuriken catapult to a shuriken cannon as it would be a waste of points. There is no point in having a shuriken cannon because only one weapon can be fired after the Fire Prism has moved up to 12".

The Fire Prism should be kept moving for two reasons. The first being, that if the Fire Prism moves further than 6" and it gets hit, any penetration will be counted as a glancing hit. The second reason, being, that it may force the enemy to move their "normal" tanks more that 6" and they won't be able to fire.

Because the ballistic skill of the guardians that operate the Fire Prism is only 3 it may be prudent to keep a Farseer close to the tank so that the crew of the tank can re-roll any missed shots with "Guide".

It is also worthwhile keeping the Fire Prism within the fire arc of a few squads of guardians with anti-grav platforms to deal with any deep striking troops that are intent on destroying the Fire Prism.

The Fire Prism can have it's twin linked shuriken catapults upgraded to a shuriken cannon.  At 20 points, it is in my opinion, too expensive and with a range of 24", it's not very useful on a vehicle that is usually positioned at the back of one's army.

VEHICLE UPGRADES:

The Fire Prism may be given any of the following upgrades, "Crystal Targeting Matrix", "Spirit Stone", "Holo-field", "Vectored Engines", "Star Engines" or "Scythes".

The "Crystal Targeting Matrix" at 30 points is a good deal because it enables the Fire Prism to fire its weapon during the movement phase. This means that the Fire Prism can stay behind cover, emerge to shoot and then return back to cover. This is an excellent upgrade to put on a Fire Prism that you are using to hold a flank as it will enable you to exploit the Prism cannons range.

The "Spirit Stone" is a fairly inexpensive upgrade that can help to reduce the effects of "Crew shaken" or "Crew stunned" results.

A Holo-field can help a Fire Prism survive longer but it is probably a waste of points. If you are smart, it will be unlikely that you will have the Fire Prism in line of sight of anything that has the range to hit it. After all, the Prism cannon has a range of 60" and should be kept at the back of the field. As a matter of fact, I’d suggest that it’s not worthwhile taking a Fire Prism to a game that is on a table that is smaller than 4’x6’.

"Vectored Engines" are inexpensive upgrade that enables you to re-roll any failed "Difficult terrain" tests. It’s unlikely that a Fire Prism would need "Vectored Engines".

"Star Engines" allow the Fire Prism to move another 2D 6" instead of shooting. This upgrade can be a useful surprise to use against an enemy in the last turn to claim a quarter or to "take and hold"

"Scythes" can be fitted to outside of the Fire Prism so if the Fire Prism gets attacked in close combat, any attacker that rolls a 1 to hit will receive, a strength 5 hit. "Scythes" can be an inexpensive upgrade that may prove to be beneficial when you are playing against an army that has infiltrating troops that are equipped to attack vehicles in hand to hand, such as Chaos.