[RPG] Changing Type Of Enethe During Combat

In the final analysis, it's up to your players to prevent metagaming -- as you've noticed, it's easy for them to figure out what's a minion and what isn't. That said:

I like to keep monster knowledge checks in mind. I know which PCs have high Nature, Arcane, Religion, and Dungeoneering skills, and I make sure the players know the things their PCs should know. For example, I'll say "oh, and Zenja, you've studied those weird Fell Taint things," because she's got a high Arcane. I don't ask for a roll, I just provide the information. This removes a lot of the emphasis on playing metagames to figure out the best tactics.

Further, on the occasions that I don't give information, the players are more likely to accept that it's a mysterious monster. They get a bit more worried, since they know it's something special and funky.

A random smattering of thoughts that are too big to fit as a comment, even though this isn't really a great answer on its own:

• If you can, use a very long battle map. On one end is where the enemy army is approaching from, and the choke point should be very near to this edge; the other end features the bridge and, ideally, a road coming from one (or both) sides. On the road is a column of refugees that each round move further along the road, across the bridge, and off that edge of the map. On the bridge have a couple of soldiers guarding the engineers who will drop the bridge; the trigger to do so is when the soldiers are engaged by non-minion enemy troops, or when one is dropped by minions. Having the engineers and the refugees in play should increase the intensity of the encounter, giving the players something concrete to protect instead of the less-so "there is a bridge somewhere behind you". Makes it more like an escort mission than a tactical exercise, but it does provide a sense of urgency for the players. XP payout could be based on the number of refugees that escape before the bridge gets blown. (As a bonus, this scenario may give you a Stage Two: If the PCs didn't get across the bridge, they now must escape themselves as they are trapped between the river and the oncoming army!) You can of course substitute other objectives (e.g. supply wagons) for the refugees, or alternate routes (e.g. a narrow mountain path that the engineers will block with an avalanche) if that fits better.
• The battle itself could be an escalating one. The army is lead by a line or two of skirmishers, represented by minions with the occasional lurker (obviously out of his element here, putting him at a severe disadvantage and keeping the early rounds easy for the PCs). Eventually the mainline troops (i.e. soldiers with the occasional brute) arrive, and not far behind them is the artillery. Soon you'll start getting HQ elements (elites/leaders) reaching the front lines as well. In this manner you can reward your party for conserving their resources early on, while presenting a believable reason for why they'd want to do that. (Probably a good idea to throw in a grizzled ol' veteran NPC on their side who basically explains what's going to happen and why they shouldn't just go nova on the first few ranks.)
• Think about enemy morale. If a unit suffers severe casualties, they may very well turn and flee, causing chaos in the other units and generally giving the PCs a little respite. Not all units should do this, though -- it'd mostly be green troops, especially conscripts who lack the training and discipline of professional soldiers. This gives the PCs an alternative strategy to simply "kill as many bad guys as you can": All they have to do is inflict enough casualties to "convince" a unit to flee; a little bit of Intimidate in here could boost this effect. Diplomacy might also work, but it would have to be a higher DC to reflect just how dang hard it is to speak eloquently during the heat of battle. Of course, this might not work depending on the flavor of your enemy army -- undead hordes probably won't flee, and similarly an army composed entirely of battle-hardened veterans will be quite stubborn opponents.
• You've already got this one it seems, but let your PCs use the terrain. The easiest route should be a narrow one that robs the enemy of much of the advantage of their numbers. However, don't have the enemy just mindlessly funnel themselves to their own doom -- intelligent commanders will order their units through more difficult terrain to try and flank the PCs, while skirmishers/lurkers will likewise try to do the same thing. The bulk of the army should go straight for that funnel, but have a few enemies take the more difficult paths around the choke point (but try to do so on the map so the PCs can deal with them, as opposed to suddenly having troops appear on the sides).
• Encourage the PCs to think outside the box -- a hay-filled barn with flimsy doors provides a very tempting alternate route to the bad guys, once they bash down those doors, but as they near the other side a PC (or even an NPC) yanks a cord that topples a lit lantern into the dry hay, immediately stirring up an inferno that engulfs the troops inside while creating a very effective barrier against others who might have followed. Depending on the prevailing winds (perhaps magically altered), the smoke might blow over the enemy army, reducing the effectiveness of their ranged attacks (or it might drift over the PCs, doing the same to them!). Or, in a similar vein, if there's a wood mill nearby the PCs could gather up sawdust, then use magical winds to whip them up as the wizard ignites it with Burning Hands or a Fireball, resulting in a huge inferno well beyond what the wizard could accomplish on his own! These and other ideas can delay and confuse an enemy army without forcing the PCs to use up much of their own resources.