Probably the best strategy with the Whirlwind is to provide support to troops from just behind your own lines. The Whirlwind's main use is to disrupt enemy units, so firing at clustered enemies can be a pain for the opposing side. This can be effective to overwatch Victory Points and to prevent infantry capping them in games.
Also, Whirlwinds can be useful to taunt heavy weapon squads such as Havocs, Noise Marines w/ Blast Masters etc. So don't forget to lay a barrage on them as well.
Whirlwinds can take considerable damage from anti-armour weaponry, so it is ideal to watch it's flanks and maybe even deploy a Scout Squad to repair it if it does take damage.