Minions aren't a big problem for Hecarim, But here's whatyou can do for each level:

Level 1:Basic Attack
Level 2:Mass Rampage
Level 3-13:Spirit of Dread, Mass Rampage
Level 14-18 Spirit of Dread, Basic Attack


My thought process behind this:

Level 1: you don't want to waste Devastating Charge on something as weak as minions, it's a soloing ability.
Level 2:Nothing else to do, and it gives fast and easy kills, as well as keeping your stacks up.
Level 3-13: It's fastest, Spirit of Dread gives you health, though not much,and keeps stacks of Rampage up. Plus, mid-way through these levels(About level 6 or 7) It doesn't waste much mana.
Level 14-18: At this point, your attack damage is high enough for just basics to kill them fast. Spirit of Dread is mainly just for it's small heal

This will not only help for 5v5. but also for 3v3:

THIS SECTION IS NOT FOR JUNGLING. IT IS USED FOR GETTING BUFFS AND EXP/GOLD FROM JUNGLE MOBS.

5 Versus 5:
Red Buff: When playing Hecarim as a jungler normally, Spirit of Dread would be the base skill. Therefore, use Spirit of Dread for an opening, Using Rampage for extra damage. Mass Rampage for good damage, or at least until the other lizards are dead. Optionally, you can use Devastating Charge, run for a little to get the extra damage from charging, and attack. It's not necessary, but can be used to kill quickly.

Blue Buff: Spirit of Dread, and Mass Rampage. Don't waste too much mana, because you regenerate mana with the buff. With low mana, it can be helpful, but is best used with high mana.

Other: Spirit of dread,Rampage Twice . Works every time.

Baron: Devastating Charge with the 3rd level, maximum. Spirit of Dread for a great health sustain, while using Rampage as fast as possible. Baron can be solo'd by Hecarim, but not until full build. So don't try it often!

3 versus 3:
Ebonmaw, Terror of Zaun: You must be the tank. Spirit of Dread and Rampage are all required, for damage and heal.

Lizard Elder: Spirit of Dread. It's all required, but Rampage a few times is recommended.