For leveling, we are going to focus on
Whirling Slash and
Lightning Spear as our main damage skills. We will be on the lookout for + Companion damage/+ minion skill level items, but in the mean time, we want to be fast and deal high damage!
As soon as you hit Level 22, you'll be able to unlock the skill
Tame Beast. Make sure you save an Uncut Skill Gem that is level 7 to immediately create this Skill Gem (level 8 will be too high to use). One of the first Unique Beasts you can tame, Crowbell, is in Act I (The Hunting Grounds). He deals BIG damage and has a massive stun build-up, so bosses are constantly staggered.
Another unique Beast you can tame is found in Keth (Act II) - the Tarnished Scarab. Once you make it to Act III, go tame Mighty Silverfist at the Jungle Ruins! This is where the FUN begins. He has HUGE damage and can become hilariously large! He has the highest critical strike chance (25% of base) of the unique beasts, so he can carry you to endgame easily.