In most cases it will be impossible to maintain the Eradication debuff, however, there are best practices to observe. Rather than aim for high uptimes with plenty of gaps, it is better to ensure that once you apply Eradication, you have enough Soul Shards to keep the debuff applied for a short period without interruption. This allows you to ensure each follow up Chaos Bolt is also buffed by the debuff. It is still optimal to stretch the debuff as long as possible, but without letting it drop.