Well, I guess I am just repeating all advice here, but anyway:
- Encounter Area Design:
Encounter Areas lead to the most interesting results if there are multiple entrances to each room. It is also a good idea to have some enemies delay entering the encounter area (or at least presenting themselves to the enemy).
This way, the enemies can attempt to surround the party - and vice versa. Skirmishers and Lurkers will try to use the "side routes" to get to the squishies. They might meet some Strikers send by the party that try the same with the monster squishies.
A good design might have the party enter an area and be allowed to explore it somewhat. The enemies might start unaware, but either way, the party only gets into combat with the enemies once they are further in to the area. Rooms are connected with each other, and there is more then one path between any two rooms. This will allows allow the option of surrounding enemies.
- Encounter Group Design and Tactics:
A group of soldiers, brutes or melee controllers can be used to stop the parties moves. Artilleries, Ranged Controllers fire from the back and target the most vulnerable targets. Skirmishers and Lurkers will try to bypass the "grand melee" between party and melee monsters.
Monsters should try to lure the party closer into the encounter area, so that the skirmishers and lurkers have room at the sidelines and behind the party to surround the party or hack down ranged strikers and controllers (Warlocks, Rangers, Wizards as prime example).
It will be interesting to note how different party setups can change the outlook of such encounters. A party with two defenders can actually build up two defense lines, so that even the enemies trying to sneak in from behind can sneak in. But a party with two Strikers might take down the initial front line so fast that the "sneaky" bastards might come a little too late (but will certainly put a lot of pressure on the Strikers to move fast)