Wind Bernard (Bernard)

Bernard (Wind Griffon) is easily one of the best three star monsters of Summoners War.
Bernard is easy to get, you can farm him with a nice drop rate in Tamor Desert scenario or in his Secret Dungeon.
It is advisable to farm him multiple times so that you can level up his skills.
With his third skill he can boost Speed and attack bar of the entire team.
Bernard has a nice two turns ATK and DEF break with his second skill.
It' essential to make him as fast as possible since in PVP getting the first turn is crucial.

Moreover his first skill's damage scale up with attack speed, so building him with Swift runes is the best choice to make.

Bernard can be used almost everywhere: from ToA to Raid battles and in dungeons like Giant B10 where he is often paired with Shannon-Wind Pixie (the duo is often called Shannard since they work really good together, providing complementary buffs on allies and debuffs on enemies).

He can be used both in Arena (and also Guild Wars) defence and offense. Bernard is the key element for speed teams: by playing first he uses Tailwind (third skill) buffing the speed all allies and increasing their attack bar by 30%. Next a mass defence breaker plays his turn just after Bernard (Galleon-Water Pirate Captain is a great option, AoE defence break plus increase attack on all allies) and finally a nuker with powerful AoE attack will obliterate enemy team   (Good options for this purpose can be Teshar-Wind Phoenix, Akhamamir-Wind Ifrit or Julie-Water Pierret).

The speed requirements on the Wind Griffon entirely depends on the environment and the intended use, in GB10 190 will be the minimum requirement for a stable run, in arena it varies in relation with your level, 250 (excluded a minimum of 14% speed totem) for conqueror 1 should be enough to overspeed all the Chloe (Fire Epikion Priest) defenses out there, for C-3 and Guardian levels 290 is the minimum requirement.

Once reached level 35 you may have doubt on six star him or even choosing between him and Megan (Water Mystic Witch),
without any doubt they have very different purposes despite they both have an attack bar buff skill, to begin Bernard has higher base speed
so he will take more advantage with less expenses from Swift runes, from speed leader units and from the speed totem, in the meanwhile he will increase the attack bar of the allies by 30% which is greater than Megan (20%) plus speed buff all the team.
TL;DR; he is a cheaper attack bar booster because he requires less speed from runes to beat Megan.

If you need to farm Dragon B10, or you are sure to take the first turn on arena, and you have already a Bernard :)
you may prefer megan especially in PvP because with her Atk/Def you can team her up with Copper (Wind Living Armor), Lushen (Wind Joker) and over nukers who will need more attack and defense
to shot enemies at the first round.

Bernard Rune Build

All games: Swift/Energy or Swift/Focus - SPD / %HP / %HP or SPD / %HP / %DEF  try to get as much accuracy as possible for his second skill, also use runes with speed substats. 

In pvp good Bernards have 290+ speed and top tier ones have 310+ speed (you will have to boost rune speed stat by using grindstones, rune power-ups droppable in Raid battles)