Would it be easier to earn xp by using certain commanders and favorable prestiges against certain maps, instead of earning the extra 25% via random? An example being Karax and Architect of War on defensive maps.
You can do Karax on Craddle of Death Brutal+6
If you have a partner willing to assist you, sure. Otherwise that strat is not viable.
Bonus exp is usually not worth the time. Some exceptions being if the map is hard timed, then often you have nothing else to do but to complete it.
It would be, although what ppl are looking for in Coop varies by person…
If someone wants to rush through, A quick mission like L&L does the trick. On a few occasions, I did that to forgo the +25% Random Bonus when I only needed 5K of XP to reach lv15, and as a bonus (pun intended I suppose), I actually never got L&L during my process of leveling that CO + prestige to lv15 (being on all Random Mission)!
Some people want the variety.
Others want the challenge, or at least some challenge. The easiest way to level up would be by just playing Normal and Casual (I’m sure there are those that do that. Hard to get easier than that!). Otherwise, you learn new things by trying things that aren’t optimal. For example, try playing Zagara on DoN. I learned being able to finish the mission 1 to 2 day-night cycles helps compensate for her inferior defenses. Otherwise, Bile Launchers can come in handy. I played a few games as Karax P0 on SoA and CoD. You learn how to properly push as Karax.
If you have the freedom to switch commander and prestige, then one question come: what do you need the exp for?
Compare the average time taken to complete Void Thrashing against the average across all maps and you’ll probably 50% better on VT. Maybe grind it for a while and then switch to something else if you get bored.