If you're missing that much it's because you're using abilities with a low chance to hit. I'd hazard a guess that you're not, and that your last sentence is slightly exaggerated. Don't worry, we've all been discouraged.
Personally, I think I see the problem in your first sentence. The minefield/gate combo, I'm assuming where you lay the mines with a mechanical pet, switch to an imp, and force a switch-in with Nether Gate, is an interesting one. Consider that as you take a turn to lay mines the enemy pet might have done anywhere from 200-500 damage. Then you switch; that's another 300 or so damage your opponent has a chance to do, and you've still done none. Then you use Nether Gate, most likely being faster than the enemy pet. It deals some damage, then the entering pet takes some damage from the mines. You are now almost caught up. But you're actually behind.
Granted, I don't know what you actually do in what order, but that payoff seems much too small. Mines are supposed to be a supplemental move, but they don't do so much damage that it's worth using several turns to ensure they get blown immediately. Meanwhile your opponent is using their own special strategy. They may be using Curse of Doom then switching in a pet with Swarm to one-shot your frontliner, or they may be using the ever-popular damage-over-time-and-force-swap tactic.
Basically, I think you should get a different setup. But if you have additional information that would warrant a reevaluation, I'd love to hear it.