The rental market on Steem Monsters/Splinterlands is relatively new, and we haven't talked very much about strategies for renting efficiently. I may do more of that in the future, but for now I want to point out two available rental cards that can boost your DEC earning rate for a very low cost, and easily pay for themselves.
Each gold foil card you use in a match gives you a 10% bonus to DEC earned in a match. But gold foil cards are expensive, especially for using in high-level play. Arianthus gets used so often that maybe it's worth paying a premium for him (and I've got a couple of level 3s up if you want them) but for most cards it's hard to make back the money you'd spend on them. The two above are exceptions, however, because they're very good cards even at lower levels in a high-level deck.
Wood Nymph is primarily useful for two abilities, Tank Heal (which she has immediately) and Strengthen, which she gets at level 6. While a high-level Nymph can make a little bit of difference, mostly it's not her attack or her HP that you're bringing her along for, so mixing a level 6 nymph, or even a level 4 one, into your Earth team is very practical. And renting her is very cheap - I just put three level 6 Nymphs on the market for two cents per day. That's just 20 DEC at current prices, so if you're using a regular Nymph, swapping it out for a gold one and getting that 10% bonus should be very profitable. If you can stand a level 4, there are many of those available for less than half a cent per day.
That's highly profitable, but even Nymph pales against Creeping Ooze. As a neutral, Ooze can be used in any splinter's battle formation, and since it costs only one mana it's easy to fit in. If you're DEC farming, you should definitely have a gold Ooze in your arsenal. Even better, since Ooze usually has only one role - to use the Slow ability - it's even easier to use at a low level than Nymph is. The main concern with a regular Ooze is getting it to level 4 so it becomes immune to Snipe, but gold Oozes start already there, which means you don't have to rent a leveled up Ooze at all. Like Nymph, there are any number of Oozes available on the rental market for less than half a cent per day - that's a ridiculous 5 DEC. And for this you get a 10% bonus to your DEC earning rate every time you use Ooze in a battle. Even a Silver player should expect renting a gold Ooze to make them more than 100% daily profit.
To rent cards you need to use the Peakmonsters market. In any card listing click on "View Cards" and then "Card Rentals" to see what rentals are available.