Here's a really simple way of balancing something:
If something is good at offense, don't make it good at defense, and vice versa.
5 ATK for 6 is good/decent. Less so upped, but decent.
I would give it more HP but less attack.
This philosophy of balancing is not necessarily applicable to all elements. Some elements will favor strictly defensive or strictly offensive creatures, some will favor both types but only as extremas (which you are suggesting), and some elements may instead favor multitasking creatures (ones which can do either reasonably well but excel at neither). Still others may prefer cards that can be shifted from one mode to another.
This is a core of elements game philosophy, each element embodies a different style or motif.
What works well in one element may not be suited to another.
This is also what makes card design for elements interesting and unique. Each of the 12 elements embodies a unique motiff, diversity in card design is strongly encouraged rather than uniformity... While uniformity is certainly easier to handle in terms of balancing and design elements chooses instead to aim for dynamic balancing.
I think a better way to approach design / balance is to think about what characterizes the element the card is being placed in.
In the case of earth, balancing is a little tricky. It seems to favor the philosophy of shifting / digging in. I.e. it has cards that start out relatively vulnerable and have a high offensive capability but that can be shifted to more defensive roles if needed. This is emphasized by its burrow mechanic and plate armor which can provide defense to a previously vulnerable creature.
Alternatively, earth can also go more the route of gravity's motif. Difficult to kill but relatively low attack to cost ratio. This is evident in stone dragon and hematite / steel golem.
With that in mind think on this:
Firstly, which of the thematic motifs would you like to embody? A creature that is vulnerable with High offensive value that can get "dug in" if threatened, or a creature that is relatively weak offensively but is very difficult to bring down relative to most other creatures of the same cost.
Next:
-If you are going for the digging in route, how can you make this creature adaptable? I.e. how can it "dig in" if threatened
-If you are going the stalwart brick wall route, ask: is this relatively balanced in terms of ATK vs Cost when compared to Stone Dragon and Hematite / steel golem... How about its "effective" toughness
From what I can tell it seems like you are actually geared more towards the tough but relatively low offence route. So we should balance relative to hematite / steel golem and stone dragon.
Since your current design hard locks HP at 3 / 4, balancing becomes relatively doable in terms of an "effective toughness"...
For effective toughness think of it this way: Assume an "average" CC damage... say 3, then figure out how many HP it would have by how much damage it would soak up if it didn't have the defensive skill protecting it.
In the case of 3 avg CC damage, the unupped version has an effective HP of 9.
So is a 5|9 for 6
on par with hematite golem and stone dragon cost efficiency wise?
