Use heavy shield infantry for the front line if you’re having issues. regular Medium infantry can’t really do attacking or defending amazingly but can do both well. however leader specific medium depending on tier level and just the unit probably do better than the regular variant.
Most units in this from what i can tell use shields, so it’s up to you if you want a lot of range or not a lot. I’m playing as hecter this campaign and the trojans tier 3 archers do alright in melee but alas not get, so i bring enough to deal with enemy rangers but can act as reserves.
For you flanking maneuvers from what I can tell, the 2handed spear infantry get a pretty good charger bonus. They’ll probably get melted by ranged until you unlock the armored versions.
Although several spear units get bonuses towards sword units, the ones i’ve compared the sword units still come out top damage wise, some up to 30 more, so then I imagine it’d come down to defense/armor. Also the skills for my heroes always had a sword infantry damage bonus such as 10-15%.
Rather than rely on a second army invest in spies. They are incredibly effective at shredding armies, garrisons and eliminating the lord leading an army. Have 1 spy hit a full 20 stack army sitting in a city, the second hit the city garrison. Even 30+ units cut down to ~25% health stand no chance against your full stack in a siege.
For your army look for high defense shielded units to hold the line and don’t let the AI flank you. Army comp depends largely on who you play as and what are available. Take Paris, I go for 6 medium to heavy infantry to tie the enemy down, 4 slingers to engage the enemy ranged units, a pair of archers, 4 Sirens (i think its sirens, the javelin using special units from the island to his south) and then fill the rest in with any unlocked special units (got a cyclops from a quest one run!) or more medium infantry.
With Odysseus early on i went with 8 of the light sword guys with a ranged attack (Sword Skirmishers maybe?) his starting medium or heavy spear unit (forget which) then grabbed 5 slingers (replace these with special units as you can, such as Giants) to absorb enemy range fire and 5 of his special skirmishers for flanking (basic javelins until you unlock these). The main battle tactic is get the enemy to engage your line head on while your infantry fire their weapons, have your slings shoot at their ranged units after the infantry meet and your skirmish units circle around (or hop out of the brush/woods to spring an ambush) for some rear shots with javelins or rear charges.
Regardless of army comp the basic campaign tactic applies: Spies do the heavy hitting so almost every battle i fight is against a greatly weakened army and/or garrison.
More campaign advice.. Always ask for resources, even from the start of turn one you can pile up several thousand with most factions before making a move. Again i will use Paris for an example. Start off with signing any “positive” deal for resources one at a time. Let’s say your brother Hector is willing to sign a Defensive Alliance on turn one and it shows positive, squeeze as much wood out of him on this deal as you can. Then after finishing that deal talk about a Military alliance for even more resources. You can gain more out of individual deals than you would by skipping straight to the Military alliance, and the same applies to NAP or Military access agreements. Squeeze the AI for every bit of resources you can regardless of your faction.
One more bit of campaign advice, if you plan to KEEP a region don’t build the buildings that reduce influence in return for resources. Instead aim for resources with a bonus to resources for higher influence, there are two versions of those in settlements. Of course if you’re not sure you can hold it long you can get the -growth + resources buildings or the +resources -influence versions. They do help short term, but long term are not as effective.