Here is a battle schematic I often use for deployment and positioning of my army:
There are only four units -
•Draigo, Librarian & 5 Paladins
•5 Paladins w. 4x psycannon
•10 GK Terminators w. Thawn: 2x psycannon
The Crusader is placed to cut off enemy LOS to the rest of the army. You can see the critical angle which I have designated as the Greek character Alpha. Strategic deployment versus most armies can use the Crusader to block enemy shooting and|or provide a 3++ cover save for the rest of the army. The Grey Knight terminators are a screening unit. Any enemy units to the left of the center line (dashed) are screened off while any enemy units to the right will bear the full brunt of your shooting. The three squads of infantry are all positioned so as to not provide any intentional cover from each other.
Note that this is an excellent foil versus Grey Knight MSU style armies which will put you in a good position to win mirror matches. My record versus any other Grey Knight army is all wins. So far I am also undefeated versus both Blood Angels and Space Wolves. The only army I have a problem with is mechanized dark eldar due to their high mobility and the ability to spam S8 AP2 shooting - mechanized dark eldar can easily ignore the Crusader.
The army I have shown here can move towards the enemy deployment zone over the course of a game while yielding a withering amount of midranged shooting each turm via psycannons and stormbolters - note that I'm a huge advocate of psybolt ammo. Horde armies are easily dealt with using the combination of shooting and melee.