Need help building a full team

Hi all,

So a friend and myself have decided to have a battle in a month or so, we have access to 6 generations but no mega's, i've never tried to put together a strong strong team only ever fun ones

the first 3 i have but were only fun ones while i play are
Hydreigon
Swampert
Electivire

We're both complete beginners, we're mostly doing this as a throw back to when we were younger the last games i played were Blue and Gold, then nothing till now.

Any recommendations?

Thanks in advance!