Some things relating to defense:
1. Shell and slime should be able to be built based on location. That is, protect all over, just your front, just your back, your sides, etc. Shell/slime built in this way can't be "moved around". That is, wherever you build it, it stays, unless you unbuild it, it gets destroyed, etc.
2. Shell and slime should have a maximum constructable amount per turn that's based on something the bot can set. Body is an obvious choice.
3. Shell/slime effectiveness is based on the surface area trying to be protected. Larger bots need alot more shell/slime to protect them than smaller bots.