This is my favourite event in SoS, especially when it aligns with events like Circuit Repair. It's better to buy pickaxes at the start as you've done (while it only costs 1 pickaxe per tile smash), get through as many levels as you can without using your magnifying glass (save them for later) and then clear later levels by redeeming your coins for bombs (which refresh daily) - saving and using bombs on bonus levels will net you free pickaxes (they never give you enough to clear it, it's a mechanism to make you lose more chances). Idling is the best thing you can do. I usually get to level 35-45 and I only need to spend £10-£12 to get there.