Try to unlock and have one of each type of monster (Attacker,Defender,Looter).
Try to unlock D.A.V.E. first if you have a level 4 Monster Locker.
Also don't spend all your putty on your monster locker!!! Use some on your monster academy too.
Once you unlock every monster, technically you can recycle it (unless you want monsters in the future that have not been released).
Skip a weak monster like bolt and try for eye-ra or even ichi.
When it's level 3 or 4 hide a defensive tower behind it.
Monster lockers aren't important at all once you have unlocked every monster. Use as part of NEC (Never Ending Chain)