Frontline: Clock, Timber, Techies, Alchemist

Backline: Tinker, Bounty, Razor, Maiden, Keeper

Synergy: (6) Goblins, (4) Mechs, (3) Mages, (2) Humans

Mages are arguably the best combination with Goblins in the mid-late game because of two reasons:

Tinker, Bounty, and Clock deal magic damage with their ults. This means the (3) Mages bonus benefits them a lot (especially Tinker). Moreover, Goblins lack damage in the mid game, which makes units like Razor very valuable. In the late game, your main source of damage is the AoE nuke of Techies. Mages give you even more AoE, which helps you wipe the whole enemy team.

If you get Crown or Void Stone, make sure to place them on your Tinker and to move him to the second line of units. This, combined with the (3) Mages bonus (and ideally Crystal Maiden), will increase his DPS a lot because he will be able to cast his Missiles multiple times per round.

In this comp, you have 9 Units. Usually, for your 10th you’d want Tide or Medusa for some CC, but if you don’t think you need control you can add more AoE damage instead (e.g. Lich or Gyrocopter).

Bear in mind that Techies deals physical damage with his ult, which means getting the (2) Undead bonus in the late game is not bad. You can swap one of the mages for a Lich and add Necro/Death Prophet to get it.