Power Ups Weapon Upgrades

[TommyTwoToes]

I am not able to upgrade my weapons any further. Have finished all planets in Serious Mode and achieved full gold stars.
Skill tree is completely unlocked. A lot of my weapons need more upgrading but have hit a wall as not able to upgrade my remaining weapons any further. Went through and finished the planets again but no further weapon upgrades are offered. How or where do I go to further upgrade my weapons to the max?
Thankyou for your time.

[Eagleshadow]

You can get further weapon upgrades by playing Arenas. Each arena difficulty level earns a single upgrade, meaning if you immediately finish lvl 20 difficulty of any specific arena, you'd get 20 weapon upgrades at once. So 8 arenas means 160 upgrades to go! You might want to start around lvl 10 difficulty or so, given that you managed to beat everything on serious.

[TommyTwoToes]

Thankyou for your reply

[FSMcas]

But as far as I understand him, he has the same question I have right now: Yes, you can get the smaller triangle upgrades in the arenas. But while you can unlock the full skill tree, you cannot upgrade all weapons, correct? A few of the big upgrades have to stay empty, or am I missing something?