If the problem is your combination of chassis, weapons and gadgets, then be creative with things - it can pay to use more than one type of weapon or gadget on your build; I tried using two Blades and ended up getting nowhere, and when it became Blade and Rocket, I powered up the ranks. Don't be afraid to mix it up. Also, make sure to match gadgets to particular weapons and chassis. Using a Backpedal on Titan tends to pull it to the ground - not overly useful. When in doubt, a Lifter or Repulsor at the front combined with either a ranged weapon or a Blade tends to work decently well. Finally, a bunch tips: ~ Boulder and Titan chassis' are the easiest to knock over. Notably, having either large wheels at the back of the chassis or driving into an enemy's Repulsor are particularly effective ways for this to happen. ~ Lasers are powerful, but slow. You can shoot off two Rockets/Double Rockets before any Lasers will be able to fire. ~ Sneakies are highly adaptable, but are very vulnerable to forklifts - unless it has a forklift too, in which case it's really a matter of brute strength. ~ Using the correct wheel combinations are important. You don't want either too high or too low ground clearance, and you should always have decent pushing power - enough to at least prevent you from being pushed back. Typically, two different but similarly sized smaller wheels can produce this effect with most chassis models. ~ In Championship mode, scoping out your opponents is a wise thing to do. If there are those opponents that you can never seem to beat, analyse their build - can you make a similar design with enough strength and health to beat it? Do you have any simple tactics that can exploit any potential weaknesses? Does it only use one type of weapon? Remember - this is war, so fight dirty. ~ Lifters are quite potent, and using two will be able to flip about anything. This is a strong tactic, but blades are very good for countering this. ~ When using shotguns, make sure you can move forward at least as much as the recoil pushes you back - otherwise, you'll blast your way out of the arena once the side walls come down, giving non-moving opponents a free victory. ~ Should you obtain a legendary part, analyse whether or not it may have use at the high end of the leagues: Legendary chassis are the least useful, as once they become obsolete in terms of health they tend to be more con- than pro-. If any other type of part gives a health boost, check which chassis it boosts, and whether the part forms a good combination with the chassis. (A Legendary Bigfoot for a Titan Chassis is a definite no-go.) If a weapon or gadget gives an attack boost, be sure the type of weapon that is effected is good enough, and if the weapon itself has the attack boost, it should be for a good chassis (Double Rocket on Standard, yes. Chainsaw on Diamond, not so much.)
If you've gone through all of this and decided that the legendary part may have use at the highest leagues, DO NOT use any toolboxes on it until you reach Stage 24. The only exception to this rule is if the legendary part is a chassis, you can use power toolboxes on it before S24. Oh boy... This took a while.