You get Ultraspiker in Roblox Tank Game by progressing through the tank upgrade path and unlocking the required higher-tier tank variant in the lobby shop; related guides and gameplay videos point to a “spiker” upgrade line and to higher-level unlocks being bought with Diamonds after reaching the needed level.

What to do

  1. Level up until tank upgrades open, which starts at level 15.
  1. Earn Diamonds by playing, since higher-tier tanks are bought with Diamonds in the lobby shop.
  1. Unlock the Spiker-related branch, then keep upgrading through the stronger variants shown in recent gameplay guides, including RangeSpiker and AllSpiker references.
  1. Check the Tanks Shop in the lobby for the exact unlock requirement, since some late-game tanks are gated behind shop unlocks at higher levels.

Extra note

Recent community posts also mention missions and rewards as part of unlocking top-tier tanks, so daily missions may help you get there faster.

In plain terms

If you are trying to reach Ultraspiker specifically, the likely path is: play matches, earn Diamonds, unlock the Spiker line, then buy the next upgrade tier once it appears in the shop.

TL;DR: Grind levels, collect Diamonds, unlock the Spiker upgrade branch, and buy the higher tank in the lobby shop once it becomes available.