MArcomage

This card is too powerful for its cost. Come on - even if you survive oneshot Undead cards like Undead host, Corpses behemoth, Ice revenant or Spectral Dragon, you probably will be dead next turn if you haven't your own oneshot or rare "OH SH~" card.
It can give you 35 wall, 23 tower and inflict 35 damage for 1/1/1 resources and all you need is to play Undead host. I can compare this card only with Mountain king that can give you 32 wall, 24 tower, 8 stock and inflict 60 damage. But 1) Mountain king is rare and Eternal Guardian is uncommon, 2) Mountain king costs 26 gems/26 recruits, 3) Mountain king needs full Alliance hand.
I don't know what to do with Eternal Guardian, to be honest. The only thing I can suggest is to remove damage from it and raise its cost to 3/3/3. Or rename it and remove its tower/wall gain. But this card definitely needs some changes.

Maybe discard with a common banish card?

...that can be not in hand, can be not in deck because it doesn't fit deck idea, or can hit another Eternal Guardian/Hungry Dead. Uncommon cards isn't hard to draw, especially if it is Undead uncommons.

There's a lot of Banish cards around nowadays and it's easily discardable.
I haven't encountered a deck that doesn't have Banish cards in it yet
Plus it's vulnerable to Holy, plus it's situational.

I could go for a 3/3/3 cost, but anything other than that will just destroy this card.

Same logic you show here could be followed for the combo of Devastator + Warlord, and a lot of other cards. Hell, I could brag on how Mountain Sentry is an Uncommon that can build max wall in no time.
Let's not make the mistake we did with Nature again.

Warchief isn't Durable, Eternal Guardian is. Warchief can give you 15 wall, Eternal Guardian - max 45 wall/30 tower. And that Barbarian combo is actually more expensive than Undead because of Undead token effect and recruits only cost of Barbarians.
Yes, Mountain Sentry is very effective card, but it needs full Alliance hand, 4 turns and 36 stock to build max wall in normal mode (6 turns and 54 stock in long mode). And Eternal Guardian doesn't need to be spammed like Chaos Dwarf or Mountain Sentry. Play Corpses Behemoth or Undead Host, if enemy still alive - play Eternal Guardian one time. You'll get lots of wall and tower, enemy - lots of damage.
In fact, leave its effect as it is, but get rid of Durable, make its cost like 8 gems/8 recruits and add Undead summon as secondary effect.

This card WAS 3/3/3. It was reduced to 1/1/1 a few years ago because it was too easy (and worthwhile) to play this card indefinitely.

I wouldn't change it, as it is now. It really is very easy to discard, especially considering it's Uncommon and there aren't any other non-rare Durables that work so well in an Undead deck. Besides, as effective as it is (and the numbers it grants you really aren't all that great unless you've just blown all of your resources on an expensive card), it seems that the rhythm of playing Undead has this card uselessly triggering the keyword effect more often than not.

Because of that I suppose to get rid of damage. Or rename it to "Eternal jack-of-all-trades".

I do not think it is at all overpowered. It definitely is a useful part of an Undead deck, but quite balanced for many reasons, some given above i.e.
- Discardable by Non-Common Holy
- Discardable by Any Banish
- Discardable by Monster Slayer (?)
- It has potentially no effect and requires expensive cards to be played to be effective (negating a lot of the advantage of being cheap)
- It cannot be effectively spammed ad infinitum (unlike a lot of Durables *Koschei cough cough Byakko cough*)
- It may have a lot of effects, but its power is spread out and thus less valuable [ "-10 enemy wall, 10 attack" should be considered better than "+10 wall, 10 attack"].

EDIT: Also, Durable is not always a useful keyword. It prevents you from drawing another card that would help your Eternal Guardian build/attack/whatever.