Fun game! It took me a bit to find a winning strategy but I got it in the end! It's fun to try and find a balance between playing too risky and using all your bullets, but also you need to be somewhat proactive in clearing out space since if the balloons pile up they start to spawn inside the pack. What I ended up doing was prioritizing clearing as many virus balloons as I could depending on how many heart balloons spawned (since 1 heart = 2 virus)!