Personally, I would recomennd purchasing Guardian over upgrading Arachnos to 20 energy. Arachnos is not really a good mech outside of early game as his ability does almost no damage to enemy mechs, and he is pretty flimsy even at high level. Javelin 6s are pretty good, especially if you have really agile or tough mechs to put them on. Rocket moarter 8s are pretty bad as a general rule, just because the don't have a huge radius, and their damage is low compared to the 10s or 12s. I don't want to cause you any problems, but you might want to take this information into consideration before upgrading Arachnos or buying Guardian, Javelins, or mortars. I hope this helps!

If you move slow you get bombed in open levels... late game 18-km mechs are almost useless in open levels, they have to run mortars and if there is no place to hide from the other mortar users🔥🙉🔥