[RPG] How to communicate, in game, to a player that they are behaving sub-optimally in a specific section (that was designed for them)

I had this problem at first too. Here are some good uses of minions.

1. Archers. Long range archers. This was the first effective use of minions I found. Make it so that rushing over to kill the minions is a time investment. Better yet, keep the minions separate from each other so that each individual minion is a time investment.
2. Terrain. This is a corollary to archers. Minions cost XP. Terrain doesn't. If you put your archer 8 squares away, the PCs can get to him. If you put your minion 8 squares away, in a tree, with a moat, the PCs can't.
3. Don't use minions for damage. There are a number of minions that invoke penalties just by being present. Wind Stalker for instance slides creatures that start their turn adjacent to it.
4. Or use them for lots of damage. The elemental spark minions in particular can explode when they're killed to hit a burst of PCs. Position them correctly and the players won't be as interested in killing these guys so quickly.
5. Use higher level minions. This is counter intuitive since I started out filling up the leftover bits of XP with minions. But investing XP to get some good minions can be worth it. Minions that are a couple levels higher than the players, particularly if they're soldier minions, will be harder to hit and they'll be capable of hitting the players. When I started using minions that were several levels higher than the party, the PCs started assuming that the normal enemies were actually minions because they seemed weaker by comparison.
6. Misdirection. This relies on 5. If your players don't know which enemies are minions and which aren't you can trick them into wasting their encounters, dailies, and action points to pop minions. Using minions that are more powerful than the NPCs (as in 5) is one way to do this. Another is to throw in one or two unique minions, instead of just tossing 4, 5, or 6 identical guys down there. Give your unique minions good minis and a bit of description. Not only will the minions eat the party's good powers, this will also keep the real boss alive longer to take more actions.

In almost all situations, improving your chance to hit is better than improving your damage roll. And thus, Attack Advantage will almost always be preferable to Damage Advantage.

I went ahead and wrote an AnyDice program to compare the two, and if you'd like to go fiddle with it, you can find it here. In the program, I compute average damage per attack for both Attack Advantage, and Damage Advantage.

I ran multiple tests with different weapons, AC bonuses, and Attack Bonuses and came to the following conclusion.

If the target's AC is more than 3 points higher than your Attack Bonus (which is usually the case), then Advantage on Attack Rolls yields a higher average damage than Advantage on Damage Rolls does. This holds up for all weapon dice-sets that exist in the PHB.

So, to give one example: that of a character with 20 STR and a Longsword...

Opposed by an AC of 15, if his Attack Bonus is +11, his Average Damage Per Attack will be...

• Attack Advantage: 9.72
• Damage Advantage: 9.45

If you increase his Attack Bonus to +12, then...

• Attack Advantage: 9.84
• Damage Advantage: 9.99

This pattern holds true as you increase AC...the larger the gap between AC and the Attack Bonus (and, in general, there will be a significant gap between the two) the less useful Damage Advantage will be.

This also follows logically. Advantage on a Damage Roll increases your chances of doing a little more damage. Advantage on an Attack Roll increases your chances of doing any damage at all. So Damage Advantage can mean the difference between doing 6 damage and 8 damage. Attack Advantage can mean the difference between 0 damage and 8 damage.

That being said, I discovered another situation in which Damage Advantage holds up better. If you do not have the two-weapon fighting feature, and so your off-hand damage does not gain the +damage from the attack stat, then the AC/Attack Bonus margin increases to 5, instead of 3. i.e. +10 to hit vs AC 15 with an off-hand weapon (no bonus), Damage Advantage is better. But, even here...it won't be often that you have such a high Attack Bonus against something with an AC that is low enough you only need to roll a 4 to hit it. So, practically speaking...this doesn't matter much.

So, TL;DR: Damage Advantage is almost always inferior to Attack Advantage.