2|6 should be balanced for this card.
2 (attack) + 0 (low HP + Earth bonus) + 5 (Guard [4]) - 1 (1 Turn Cool-Down) + 0 (Burrow) = 6
2 (Attack) + 1 (Medium-Low HP) + 5 (Guard [4]) - 1 (1 Turn Cool-Down) + 0 (Burrow) - 2 (Upgrade [7]) = 5
Text should also read:
: Burrow
0 : Guard (Unburrow)
Enters burrowed.
Note: "Enters burrowed" means that the card stats are what the stats will be 2|6 while it is burrowed (compare to Graboid). Therefore, when the unburrow effect takes place, the Tank will have 4 attack, and also the 'guard' effect will also be 4 damage.