The Algerians started rushing troops earlier than anticipated. I hadn't even started getting military units out and they had archers(non-mercenary, which Algerians have) halfway across the map. I, playing as Austria, quickly pump out some Roundshiers(Austrian unique unit) to meet them. They're cheaper, and have four times the defense against arrows(archers are the only unit in the game using arrows) than pikemen do. I get about 20 or so against their 16, and I lost nearly half killing them all, most to the initial barrage. Right, that works. I start churning them out nonstop.
I'm able to grind forward this way to about 2/3rds of the distance between my town and theirs, and even them adding Light Infantry to their lineup didn't help much; I don't recall taking more than single digits' worth of casualties across the entire game from the LIs, and even then it was probably the archers. They did, of course, delay my Roundshier wall from reaching the archers.
Started researching unit upgrades from the barracks. Discovered that the physical training upgrades improve the defense stat of
all damage types for Roundshiers and Pikemen, and gave a significant advantage. Eventually got the stables built, plus the cavalry training time upgrade, and put out a good-sized force of Croats(also Austrian unique - nice light cavalry). Using them to push the initial wave of archers devastated their army, and allowed my troops to rampage through their town. Before long, they surrendered.
Cossacks 3. Despite a rocky release, they've been dropping patches pretty often and it's very playable.