(if something like this has been discussed previously, I apologies)
Basically a parasitic lifestyle of reproduction,
In microbe stage you could have an organelle that injects a spore or DNA into another cell that will force the cell to produce more of your which will eventually kill it, like viruses.
In Multicellular+ it could work like parasitic wasps do, by injecting eggs in to hosts that will hatch and become larva that eats an organism out, matures and repeats the cycle.
also thrive has progressed well, i can ply it with pretty good fps!