Akki Underminer... overpriced, right? For perspective, just one more mana can buy you a Greater Harvester instead, which is much less a 1/1. Still, the Underminer is pretty unique - nothing else quite does exactly what he does, serving as a sort of proto-Annihilator. So even if he isn't great, how do we make him work?
Evasion is definitely needed - how likely is a 4-mana 1/1 going to swing and connect? Thankfully, Goblin Tunneler is in-tribe and can get the Underminer in to do his thing. Break Through the Line requires mana, but can grant evasion to multiple goblins at once, is harder to blow up, and even provides Haste if you have the mana to spare.
Too slow at 4 mana? Goblin Lackey and Warren Instigator can both hurry out the Underminer without using up your mana to play a Tunneler. Luckily, the Tunneler can also target either of them, ensuring you aren't stuck unable to connect with a Lackey while you have an Underminer in hand.
If forcing a sacrifice is good... wouldn't two be even better? Embercleave can give the Underminer double strike, upping him to a more-respectable four damage and forcing two sacrifices. Destructive Urge also doubles down, forcing an extra sacrifice of a land in addition to the Underminer's usual tribute. Hearthfire Hobgoblin, with his built-in double strike, is also a perfect target for Destructive Urge and is still small enough to be targeted by Goblin Tunneler.
1 or 2 points of damage at a time too slow? Goblin Burrows can apply a power boost after your Tunneler or Break Through the Line have applied unblockabiity for the turn, earning the best of both worlds. Goblin Piledriver can be made unblockable, then swing for huge amounts of damage, finishing off an opponent who is still struggling to get their board set up.
