Okay, so you have two mercs and pretty much those are the ones you also use for your dungeon grind and stuff. Your primary objective is to boost them as soon as possible, sacrificing other mercs and using all the porings you got from gold chests, draws and missions. So now you got, say, two level 75/99 mercs and when you go to battle, you find it easy to win especially if you're just battling with low level mercenaries. But then, you suddenly find yourself an opponent with strong mercenaries and with the first battle, you lost. What do you do?
If you're not much of a hardcore when it comes to mercs, you'll just go with it and hope that you won't battle any more of those strong mercenaries. Your win-loss record don't matter much. But if you are concerned with your mercenary ranking, it's a big hurdle you need to address. What to do if the opponent got the upper hand??
1. Recognize other mercenaries - it's not enough you know your mercenaries. it's essential to also gauge what the opponent's mercenaries can do. You can look at this helpful link for more info about the mercenaries: MaxHP, def, matk, atk, mdef, aspd etc.
2. Switch it up! - Okay, so you lost your first battle with a particular opponent. If you lost by a huge margin, will you try again hoping that you will win? NO! Coz you'll just lose again. It's not like your mercenaries will suddenly get stronger the 2nd time around. Do something about it. If you're paying attention to the battle, you could may be try to switch the position of your mercenaries, see if there will be a significant change. I always use this technique if I find myself losing the battle. And it works most of the time because sometimes, an opponent's mercenaries may be strong against yours but a switch on your end may do the trick and suddenly you're winning.
3. The more the merrier - Don't get too complacent with how good your current mercenaries are in the battle. It is best to have options. So continue on gaining and compounding mercenaries, leveling different kinds of mercenaries. Find the right tandem to battle particular opponents. When you find yourself at the top of the rank, it is much easier to determine your opponents' tactics as you have battled them before, unless they introduce something new, in which case you would be ready as you have already developed some mercs to counter new breeds.
4. Level up the skills! - We can level the mercenary skills up until Level 3. If you got the same mercenaries as to that your battling with, its the skills that will win you the match (and a bit of luck). Don't just sacrifice mercs randomly for any of your favored ones. Look at their skills and see if you can use them to level up some of your reserved mercs' skill.
I got awarded 210 rubies last month for placing top in the mercenary ranking. I'm not really sure if that's really for that particular feat I achieved but my guildie reckons its for that so I'm quite happy.
*Btw, i think you can only battle players within 50 points of your LP. So if you got 1500 LP, you'll likely battle those within 1450-1550 LP range.