I’ve not used 5 of the same troops when fighting on the app. How should I pick which troop type to use? Is there a general rule/concept you could share given you are normally fighting a mix of troop types?
Well, really it's a whole new thread and a half. But my view is:
As a starting point look for the one that gives you a bonus against as many enemies as possible and deficit against as few as possible. It's probably easier to explain with an example.
Imagine you are facing 2 thieves (Light Melee), 2 thornrose (Mage), 1 Mortar (Heavy Range). Not an unusual combo in one of the provinces IIRC, and not a particularly simple one. If you use:
LM you have a bonus against the 2 mages; the HR has a bonus against you: so you are up against one more than you are down against.
LR you get a bonus on the 2 mages, but are down against the other 3: net down 1.
Mage: down vs 2 LM, up vs 1 HR and equal against the mages: net down 1.
HM: up vs the HR and 2LM, down vs the mages: net up 1.
HR: up vs 2 LM, down vs the 2 mages: equal each way.
So both light and heavy melee get an advantage against more enemies than they suffer against - but only slightly.
The next thing to consider is if any are enemies are specialist. Although all units get a bonus vs 2 types, some get a sensible bonus vs both and some specialise against one in particular - in the app this is about the only info it gives you: they show more stars against one than the other. Generally the specialists do get a pretty decisive advantage - even one of these against you can be problematic, 2 is suicide.
In this case the 2 enemy LM are generalist. But the thornrose are specialist versus heavy melee. Those 2 thornrose would possibly defeat 5 of your HM by themselves without any help from the other 3 units.
Next I would say: generally ranged troops, especially LR and mages (not quite so much HR), perform a lot better. And to a lesser extent fast moving units. Short range and slower units get better stats - but probably not enough. So you want to favour using ranged; and also worry more about the enemy ranged. Which doesn't help so much in this case as the initial suggestions were both melee. Although having a unit that kills those enemy mages is a good thing - which again favours light over heavy melee.
And related to that - consider if you have any Enlightened Light Range or Mage Multiplier buildings in use: at +50% damage each these make a dramatic difference. With 2 or 3 of them at once you're using that troop in all cases unless the enemy combo is awful for them.
Next... if you have decided a type you probably have a choice of which specific one to use: barracks being generalist and the other 2 specialising. If you go with LM there would be no point using drone riders as they specialise vs LR - of which the enemy has none. Which leaves your basic sword dancer/barbarian, or the hounds (cerberus). One big consideration unless you are chapter 15+ is how upgraded they are. More upgraded troops are definitely better; especially 3* troops can be substantially better than 2* as they impose a penalty (varies, but can be things like -20% damage for 2 rounds) on any enemy they hit.
In this case hounds look to make sense as there are 2 enemies they specialise against (and no LR which is their weak bonus) - and those 2 units may be the most significant enemy ones.
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I think beyond that you get into the specifics of how the battle will play out. Which you can to some extent figure out from the wiki, but is much easier if you play in the browser at least some times to see it.
For these opponents: the enemy mages have 2 movement and go first (and will hit your troops). The thieves have move 3 and go next, so will likely move in front of the mages. And the mortar has 'infinite' range and will sit at the back sniping away - but its stats are fairly weak, so won't be decisive in the short term. So if you use melee troops they may tangle with the enemy LM first, while both the mages and HR are behind firing at you. This isn't great.
Which is where your basic LM and the hounds diverge. Your sword dancer/barbarian will move second after the thornrose move and shoot at you, but can't reach them. Then the enemy LM move between you. Next round your LM are likely to tangle with the enemy LM, with his ranged are hitting you. Once your LM kill his, they've then got to chase down the various enemy ranged units which may be hiding at the back (or not - the AI can be dumb!), which may take a few turns as they're not that quick.
OTOH if you use hounds they can run clean across the map. They have excellent bonuses vs mages, so won't suffer much in the first hit they get. Then your hounds will run past the enemy LM and kill those mages in your first turn - possibly only needing one hound each to do it. Now you're fighting 5v3 and the fight will rapidly tip in your favour. And your hounds can close in on that enemy HR in a single turn once they get a chance.
Part of this is specifics, and part general experience: eg basic LM tend to underperform. Hounds vs mortar is not too bad. etc.
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So, would I use hounds? Quite likely not. I'd most likely use blossom princesses. Eeek - this contradicts my very first step. However this is a rather specific situation, and that advice is generally valid.
This comes down to the exact mechanics. With blossom vs enemy thieves: the blossom princess moves first and can shoot at range 5. The enemy thief only has 3 move and 1 range - so can't reach to hit back on the next turn. So although the thief has a bonus vs mages, the blossom and human priest can use their range to wipe out that advantage within reason. 5 enemy thieves would just overrun you, but a couple of them is quite feasible. The AI might or might not play this smartly for you - but blossoms should work either way.
In this case human priests have a problem: blossom (yours) and thornrose (enemy) move first. So with human priests the enemy mages go first and hit you. And mages are fairly high damage, low hit points so that first hit matters. Priests would likely win the fight, but blossoms are substantially better as they can kill the enemy thornrose and get their first hit on the LM in round 1.
Although this is quite specific, it does also lead back to another point I made earlier: favour light ranged or mages as your pick where possible.
Note that in this battle if you change the enemy mages to enchantress the equation can change hugely. Suddenly HM becomes viable for the player, as these mages are specialist vs HR, so not so great against HM. And they are short ranged, so will move close enough HM could engage them.
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So to summarise: try to pick a troop where you have a bonus against more opponents than have a bonus against you.
Favour LR or mages, strongly so if you have ELR or MMM buildings in play. HR and hounds are next best.
Favour 3* troops if you only have some upgraded.
Never pick a troop where the enemy has 2 true specialists against it. Only go against 1 specialist if you are very good against the other enemies.
And if you want to a get a real feel for all the detailed combinations doing at least some fighting in the browser to learn will help.