I've played Rhun a few times now and they are a really fun faction to play so I thought I'd put a little guide together.
It's my first go at doing this sort of thing so any mistakes or bad advice obviously feel free to point out
Ok, so Rhun - pretty nice starting position, potential for good economy, some great units and a long, long campaign (need 50 settlements).
You will start off with some good generals and some tribesmen and clan warriors scattered around.
Move troops and a general from Rhomen and Kelepar to take Kugavod - this will be your northern army for a while so keep it supplied with a dribble of new troops. Kugavod is potentially a real money maker so develop it wisely.
Down south move all troops and general down from Upper Khand to take Khand.
From Mistrand, Burh Ermanarikis, Rhunaer and Mattaram start moving troops towards Uldonavan - take this castle as soon as you have enough troops there as it's very important.
As for your towns go for growth but try to get some stables up where you can - Variag Raiders (horse archers) will be your key to fighting Dale armies out in the open (rather OP tbh). You will also get Variag Raiders as rewards from quests so try to do them.
Around this time you will probably be encountering some Dale stacks, some may be quite large. Don't be afraid to take them on as Dale early units are quite poor and not very mobile.
I won't go into specific tactics but basically skirmish with archers, tribesmen and use generals and variags to nullify any cav they have. Tribesmen lobbing javelins at the enemy from behind your infantry followed by a supporting charge when the javelins are gone is very effective. Variags do very well vs light armoured infantry and cav and are also good for charging Dale archers. Some Dale armies are very archer heavy, in this instance what I usually do is draw their fire with a cheap archery unit while flanking them.
Ok, you have knocked out the first Dale wave now move your army up from Uldonavan to join up with the troops at Kugavod area and start whittling away at the Dale settlements starting at Dorwinion, move through the Dale towns systematically. Also keep a small army in the Uldonavan area and try to get Tirith Thoron and aid any Mordor stacks in the area to stop Dale's southern expedition.
It is sometimes possible to keep the dwarves sweet for this period as they will be fighting orcs - keep a diplomat at Erebor. If they refuse to leave you alone be aware that they usually have 2 directions of attack - one straight from Erebor and a sneaky route from their castle up in the far north-east so place towers accordingly and watch those routes.
So eventually Dale are gone, you have a nice chunk of land, a growing economy at the Sea of Rhun and the Dale towns (Riverrun, Lonsdale, Esgaroth and Dale will make good money when developed)...what next ?
Well, there are a number of ways it can go, send out some spies and diplomats, get map info and try to see how things are going in the world (toggle_fow for a snapshot if you must ).
Depending on how Mordor are doing you could help them vs Silvans.
I don't usually go all out in the north just hinder the dwarves and silvans and make sure Mordor maintains a presence but if you want to go full out on the dwarves and silvans before turning your attention elsewhere feel free.
Sneaky tip - the general/king at Erebor likes to stand just outside the town, if you attack him with a force of the right size at that position he will call reinforcements from Erebor. These reinforcements start way off the map up in the hills so if you are quick you can kill the lone general troop before his reinforcements get into the battle - bingo...you have got Erebor. (This is a rather cheap way of getting Erebor and purists may not like the idea but after constant streams of dwarf stacks hounding you while you fight Dale you will hate the dwarves so much you may feel like doing it).
Your campaign includes destroying Gondor so at some point you are going to have to move your attention south-west.
How you approach Gondor will depend on how strong they are at this time but there are several options....
1. Get a few good generals and a stack of worthless generals and send them on the long trek to Gondor across the Dead Marshes with as many troops as you can afford.
Loop southwards across what should still be Harad lands (make sure you have military access), cross the islands into Gondor and hit Dol Amroth. Once you achieve this hold it at all costs and develop it into your southern base. Send raiding parties west to take Annulond and the other towns and use your main force to defend Dol Amroth until you can recruit decent units there and are ready to push east.
2. Buy or trade Thoronburg and/or Cair Andros if Mordor have them and use these as a base to attack Gondor alongside Mordor. They will have high Melkor culture so it shouldn't take too long before you can crank units out.
3. Approach from the north via Rohan.
Expect some epic fights with Gondor for a while but eventually they should start to crumble due to the 3 pronged attack from you, Harad and Mordor. Get as many towns as you can but make sure you keep things sweet with Mordor. Don't worry if they get Minas Tirith instead of you as you can always buy it off them later.
Watch your northern borders as Rohan may attack.
Around this time Harad may be getting to be a problem as they are hemmed in, they might go to war with Mordor or you. If you have to decide to choose between them make sure you stay allied with Mordor. The reason for this is that they are a much bigger threat to your homelands and also Harad are easier to crush if you wish to pursue that line of conquest.
Now the final slog to get 50 settlements. Make sure you get your share of the spoils in Rohan and don't be reticent about going to war with Isengard if you must. Coupled with your operations in the north against the dwarves and silvans you should be able to eventually achieve 50 settlements.
The Rhun campaign can go so many ways after you have killed Dale, that's what makes it so fun along with drooling at your golden legions of doom when they take the field
Hope it was helpful.