I'm still pretty new to the game, there's a lot to learn, but I picked up a Numenor army pretty cheap.

After doing some research online about Isiludur and using the ring, it appears as though all advice and tactics online were when he could use the ring whilst mounted.

I'm hoping the ring still has some uses now that he can't use it on horse and I'm trying to figure out what those uses are.

Would someone mind sharing some tips? Am I right in thinking he should still always be given a horse as the benefits for a choppy hero such as he, outweigh the limitations of being unable to use the ring?

When he is on foot, what are the best strategies for using the ring? What do you want to achieve from it? Run straight towards the highest fight value enemy?

The ring is good of course as it halves the enemies fight value, so I would say the target is monsters for evil armies or combat heroes for good armies. Halving the fight value will save you from spending precious might on heroic strikes.

He is probably better mounted though, due to the extra killing power and mobility.

I suppose you can always dismount and then just put the ring on if needed - I think that's allowed ?

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Soon Master Elf, you will enjoy the fabled hospitality of the Dwarves! Roaring fires, malt beer, red meat off the bone!

Yeah, Scrib is right about being able to get off the horse if it really becomes necessary to use the ring. But you do lose the horse for the rest of the game.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

I thought this was neutered in the FAQs or new rule set. I was watching GBHL how to play Numenor the other day and they mentioned something about the horse, Isildur and the ring all together.

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"Diplomacy is the art of saying 'nice doggie' until you can find a rock." -Will Rogers

The new FAQ adds some lines to the Ring's rules that make a mount throw the rider if they put it on. Perhaps the Brits were talking about riding up, dismounting, and then putting on the ring to achieve some effect. Or maybe it was before the FAQ came out since the text was still highlighted purple.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

To the question of the topic I will add this. My tip for playing Isildur with the ring of foot is to play aggressively, and use the ring liberally, to hunt troop killers and lesser heroes. But I would also advise you to only play this way when you have another hero as your general so as to prevent a few bad dice roles from costing you your best piece and your general. When playing him as your general use the ring to get away from fights you can't win quick so you can use him against enemy troops. That is how I played him and he preformed quite well for me but that was before things like Bolg and Thranduil were a thing.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Yeah the new FAQ is a shame, because he used to be pretty much a staple in my armies, and he alongside Gil-Galad and a couple of warriors to create traps can take out literally anything in one turn. Now though, he is still useful and probably alright for his points, but he is less versatile than before because like you say, he has to dismount to use the ring. He used to be an absolute steal for his points, and now he is admittedly worse, but still has his uses.
I used to take him at 850 with Gil-Galad and the twins, and at 1000 with Glorfindel thrown in as well; now however I am unsure as to whether I'd rather take a Rivendell Knight Captain instead, who may not be able to play the same kind of tricks, but will kill more. The great thing about the old Isildur was that, similar to Gil-Galad, he was the perfect all-round combat hero, capable of killing troops and dealing with enemy heroes and monsters in equal measure. Now, once you use the ring to take out a hero or monster, your troop killing power, along with manoeuvrability, are gone.

PS. forgive me for thread necromancy; Haven't been on here for ages. Also get yourself on the GBHL facebook group in addition, as online activity has slowly shifted from here and TLA over to there.