Cornerstone of all Emergo strategy is getting a few strong pieces rather than a lot of weak ones.
Now ironically you cannot do that yourself: only your opponent can stack up your men, and he does so by capturing them. Having your men captured is great! The more the better! But .... you must be sure that you can liberate them afterwards.
To put it simple, Emergo strategy is:

  • Feed - Decapitate - Bury

'Feeding' is another word for forcing your opponent to capture. You may feed as many men as you posibly can to an opponent's weak piece, but you should always make sure that you can decapitate it eventually, thereby liberating a very strong piece. Now you start to bury the opponent's men under this piece, using your tactical ingenuity to get it done.

  • Pieces always start out at their strongest!
  • Involvement in the action means an increasing number of prisoners under a decreasing cap.

There's a certain moment in which a piece becomes a liability. It's easy to see that 'six under six' means a win in virtually any situation: barring exceptional positions, you can't remove the 6-men cap with the six men left.
But how about five under two? This would depend, if not on tactics, on the nature of the position. Can the piece retreat while others wear down would-be attackers? Then it's a nice catch that may even play an offensive role in a small caps endgame.
But if the piece gets drawn into a combination prior to that, it usually means that more prisoners are liberated than the number you tried to tuck away in the first place, because the piece is fed in the process of its decapitation. Things may look safe, but tactics are diabolical!

The entering phase
Strategy in the entering phase differs for white and black.
From white's point of view having a board full of single pieces at the start of the movement phase is a good thing: all pieces are weak, and looking for an initial feeding and decapitation combination usually proves successful enough for a strong initiative. This impicitly gives another reason white should not attack: if black simply allows the capture by entering elsewhere, white gets a weak piece on the board and a provisional shadowpiece of 2 in hand, but he will lose the first move in the movement phase.

Black faces quite a different situation: attack may be purposeful either way: on the one hand it may increase his shadowpiece, but the resulting combinations may also result in his taking over the first move. Of course black should attack smart: a simple block or a simple exchange should not be among white's options. Nor should an attack be postponed too long: attacks in a very crowded position are known to have backfired big time...