Panzerkeil is a strategy to create a mobile force that is highly effective and hardy. The name means tank wedge in German.
Back in World War 2, the Germans used a particular tank formation to protect them against opposing Soviet anti-tank fire. The hardiest Tiger and Panther panzers made up the front line and sides, and in the middle the less armored vehicles advanced, protected by a thick metal shield on all sides. Historically the Germans had mixed results, but that doesn't mean the tactic can't be used effectively in Supreme Commander.
The concept is very simple: position the hard to kill units at the front and on the sides; the softer units in the middle.
The best faction to pull this off is the UEF: a first line of Titans, followed by shield generators, a second and third line of Pillar Heavy Tanks and Missile Launchers would form a hard to break force. The UEF can pull this off at T2 by putting Pillars instead of Titans and using a Mongoose as their damage dealing light units, however, this strategy fails if the Mongoose units get ahead of the pillars, being faster.
Actual composition varies on situation, obviously, but as a rule of thumb "soft" units roughly are twice as numerous as "hard" ones. An example: three Titans on front, two on each side, two Shield Generators, four Flak AA, three Missile Launchers for medium-range artillery strikes and three Pillars for close range support and rear area guard.
Note: Cybrans have little to no benefit from this tactic, as it pivots on their weak spots: fragile Siege Bots and lack of Shield Generators basically kills off its implementation. Cybrans might prefer hit-and-run engagements and small, stealth squads.
Most enemy fire will be focused on the Titans, so the lighter vehicles in the rear ranks are pretty safe and free to fire on targets.
It's important to NOT include UEF T3 Artillery or Cybran T3 Artillery in the mix, because they need to stop before firing. Only the Aeon T3 Mobile Heavy Artillery can fire on the move. This formation is meant to be kept moving at all times, since it is so easy to outflank.
An exception if this tactic is used defensively: a higher percentage of Siege Bots together with T3 Artillery can be used as a mobile pillbox to block off chokepoints with relative ease.
Outflanking is this strategy's primary weakness. Fast-moving land units - or worse, Gunships - can maneuver easily around the front line and rip to shreds the relatively unprotected rear. Also, being made of tightly spaced units, it can be very vulnerable to air strikes or artillery barrages once the Shield Generators fail.