Well alone he can take 2 orcwarriors if lucky 3. He can also take down 5 goblin ambushers in first turn, so there is no need to run after them.
But I agree generally what you are saying. Orc warriors are bit too tough, it forces player to make party specialized just to kill them.If you make bad choices lvling ur brothers they are imposible to win, but when you know best builds then they are too easy. I would like to see party of 30 orc warriors.
Its huge difference if your brother is lvl 10 or 11 as usually the last perk will finish the synenergy with the build.
I like perk system now, its well thought. 10 perks is just perfect ammount to build ur brothers and there is some synenergy. Like perfect focus + battle flow. I think devs told that they will remake perk tree someday, but they are focused in adding new content now to make the world more interactive.
Only time i saw orc one shotting max gear brother was orc berserker 2handed axe headshot.
Another note, noone is forcing you to fight them.