I was consistently beating people two tower levels above me with Mortar strat. Without specifically looking at what cards you have available or understanding your meta, here's some general advice for punching above your weight.
I think what you want is cheap defenders that you can play intelligently in order to gain an elixir advantage, and then a win condition that punishes quickly. So like Hog or Elite Barbarians to win. Maaaaybe Royal Giant? Mortar works because if you drop it when they're empty on Elixir, they can't drop a distraction to screw up its targeting, it'll hit the Tower with its first shot. Balloon + Rage could fit the bill if you're really confident and good at keeping track of enemy hand cycles. Slow win conditions (like Pekka or Spear Gobs behind Giant) are tough because the other guy will have a chance to cycle into the proper counter before your win condition can do enough (or any) damage.
Cheap defenders that trade up well would be things like Ice Spirit, Skeletons, Ice Wiz, Ice Golem, Guards, Tomb Stone, even Knight. Maybe Bats for some anti-air capacity? What you want are cards that take a few hits to kill, can drag aggro, cost less than the spell that naturally kills them, and are flexible. Two-card defenses are usually good, Ice Golem to drag something off the road and tank hits and then Skeletons for DPS, for example. Just don't over-commit if you can help it or else a single Fireball will wipe you out.
So choose like three or four of those, a win condition or two, and then a couple spells. If you're super comfortable with any particular card, try and include that.
You'll definitely be playing for 1-crown victories. It's very tough to get on the enemy throne if it has two towers shooting your push at high crown level, plus the throne HP is gunna be huge. Score your one crown and then pivot to defense (an off-lane push that you expect to fail is still "defense" if it forces or tricks the enemy into defending).
Finally, plan your next upgrades. You'll almost certainly want to upgrade your win condition ASAP for the damage potential. But then understand where upgrades aren't as important. Skeletons, for example, can usually lag a bit behind because they die to one shot against anything. Zap is ok as long as it brings Minions down far enough that your Tower only needs one shot to finish them off; if your tower needs two shots to kill after a Zap, the Zap is too far underleveled.
https://www.deckshop.pro does a good job of calling out which cards can lag behind on level and which must keep pace.
