This guide is something i decided to bring to peoples attention.

The easiest way to get Marshal is to use these 4 battle objectives for each battle (raids will get you objectives faster)

Regular Battles
Defeat 3 Officers
Defeat 4 Officers
Troop Strength 50%
Defeat Specific Officer

Raids
Defeat Specific Officer
Troop Strength 50%
Troop Strength 75% (may sub for 3 bases)
Capture Specific Base

For Chaos mode you may substitute this with bases instead due to the tankiness of officers

The easiest 3 strategies to use are the following

Forming an Alliance
Requesting supplies (can sub this with raid)
Participating in an invasion

Strategist is easier to obtain than marshal since you dont have to do a single battle to obtain strategist.

If you are attempting to go for the title associated with free officer to marshal, the easiest way to obtain this is to do alot of raids and do any battle objectives (any objectives work since its only 3v3). Also finishing end-tier quests to get relations up with the ruler (assuming you didnt join his nation and up'd your relations and then left). You will need to join a battle on the side of the ruler and get them to "notice" you. You must also NOT be a vagabond.

The other title (promotion to another nation) also works with the same method as above, high relations with that ruler and simply doing battle objectives, also getting the other ruler to notice you. You will also need to be a lieutenant (the chances for this seem very low) or a common officer, you CANNOT be prefect, marshal or strategist. It is best to do this as a common officer, it appears you have a higher chance of being promoted over others in another nation.

As said before, if someone is in the marshal/strategist spot, you will need to either capture them and hope your ruler recruits or executes them or youll have to join that nation temporarily and slander them out of position (which puts you back to lieutenant and not recommended). The promotion to another nation is significantly harder to achieve due to rulers promoting people over you constantly.

Having a good relation with your ruler will improve your chances of being accepted as a marshal or strategist

If there is another person as a marshal or strategist, your only hope is to slander him out of the position, you can also enter a battle with the marshal on your side and immediately withdraw, this usually gets a random character captured (usually the marching commander) and can be a way for the marshal to be recruited/executed by the enemy, thereby releasing the position for you to attempt to inhabit (slander is more viable)