You should start off with an extra life or 1 upgrade to use before you start endless mode, I'm tired of having to buy clones 5 rounds in a row because some swordsman bot behind me deflects an arrow back at me that missed or I get knocked down by Spidertron for what feels like 10 seconds and then shot by an arrow or hit by a saw blade, or god forbid a second Spidertron shoots another grenade in your direction.
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A few pieces of advice:
1. Never move in a straight line: If you're never moving in a straight line, arrows can't hit you.
2. Never stop moving: enemies don't predict your trajectory or even account for your movement, so if you're constantly moving away from where you just were, that Spider-tron bomb can't hit you.
3. Invest in the "Kick" and "Get Up!" skills: the former is useless, except for acquiring the latter, which itself reduces your "stunned" time to what feels like 1/4, an awesome perk to have.
4. Practice your aiming, and get a bow: If you have decent energy capacity and a bow, you can down (or at least damage) a Spider-tron 6000 within the first couple seconds of a round. This is enormously helpful.
Endless Mode was meant to be difficult. Doborog's working on making it more fair, but the difficulty is something you'll just have to get used to for the most-part. Hope this helps, and good luck!