There is no clear shortcut. Follow the study guide and references topics. Mostly developers tend to skip dispatcher, would recommend to get more hands on knowledge on dispatcher. give equal weightage to CORE components and templates development. 

on top of Vijaya's req please provide info on hwo your BP-LC nodes are setup. You can also check on how the overall BP Vs LC relationship looks using the MSM Control center (http..../etc/blueprints.html) --> Provide the blueprint path under left nav selector lookup.   

do you have a CDN in place (Akamai ?) the best way to handle DDoS is through CDN.

well this seems like a datastore corruption issue, may be specific to one single blob or multiple. If you are aware of the node where in the site is looking for info, locate the node and update/upload the content.  this should restore the datastore

try checking on the Apache Sling Referrer filter where in you can restrict who get to access your AEM env. The Sling Referrer Filter {#the-sling-referrer-filter}To address known security issues with Cross-Site Request Forgery (CSRF) in CRX WebDAV and Apache Sling you need to add configurations for t...