If you don't want to spend half your game collecting orbs, then naturally you have to kill some slugs. Here are techniques to do it.
Blindly rushing in usually isn't the best idea, since there are a hundred ways it can go wrong. But your enemy just might be caught unawares and you can steal some mass afterall. Rushing is usually used on slugs bigger then you are, since if you're small then you have nothing to lose other then those 5 tiny orbs you snatched up.
I call this technicue a 'passover' because you pass in front of them, or over their head. It can be used on any slug, even while they're boosting. This technique works best on slugs your size or slightly larger. Creating a death wall in front of a slug's face will most likely give you some orbs to collect. It's extremely hard to turn in time when someone does a passover, and boosting won't really help you survive. Of course there are downsides, like if someone else swoops in for the mass, but this is one of the most lethal ways to kill someone. You just have to get close enough.
Now this one is simple. All you do is run straight through someone while boosting, they instantly die and you get instant mass. If they boost in time, then you'll be the one dieing but like rushing, there's pro's and con's of doing this.
This is most likely adapted from Slither.io, althought it's not the most effective since slime disappears quickly. Circle around another slug, usually a little smaller then you, while boosting. The smaller the slug the wider the turn, so usually they die. However, this technique is much easier to escape through the disappearing slime. Your boosting also gives you a wider turn, unless you're a behemoth, in which case you probably shouldn't do this.
Slugs that head in one direction for a long while are usually thought to be inactive, and are easy mass. That is, unless the player actually isn't inactive and is just waiting to bait someone into their trap. Since the attacking slug will usually be boosting toward you, passovers are the best way to kill. Or you could do some defensive moves.