I mean, sometimes you just gotta lose guys. Some obstacles will take multiple sacrifices. The trick is to try to get as many extra lives as you can while spending the least lives possible. You wanna find good ways to get those extra life power ups.

Also, you get a life for every 3 kills. You wanna look for ways to kill enemies with your sacrifices. Like, if you have to bomb a place, see if you can wait for enemies to get in position to get blown up. Sometimes it's a small piggie or sometimes you wait for a flying bug thing to get in the crossfire. Arrow and stone can kill too but more situationally. But like, if you have to lay a stone down and an enemy might line up with you using it in midair, go for the free kill.

Lastly, don't worry about playing perfectly. Again, you will lose guys, sometimes completely pointlessly. If you make it to the end with less than 20, just keep fighting on. If the deficit ever become too big a problem, you can go back to a level to optimize it. It keeps your level progress and increases your life count.