I have a question for how this interaction works. Let's say that I have a Retreat to Kazandu and a Zendikar's Roil on the field. And then I cast a Explosive vegetation. Can I let the two Green 2/2 elementals resolve first and then put the +1/+1 counters from retreat on the green elementals? Any info would be much appreciated.
No. When Retreat to Kazandu triggers, you have to immediately choose a target when you choose the +1/+1 counter option. Even if you order the triggers so you get the elementals first, they will not be eligible targets for retreat.
Zendikar's Roil and Retreat to Kazandu question