Nuclear weapons are dealt with in Rifts Sourcebook 4: Coalition Navy. There are bits about common ultra-low yield weapons players could get their hands on and stats for how to deal with all the various effects of a nuclear explosion, including the heat blast, ultrafast winds, initial radiation, and the EMP blast. The bit you'd be interested in would be the "city-busters" the CS reverse-engineered from surviving equivalents to the Tomahawk cruise missile. Their 200kt warheads are capable of dropping 300-1200 md each at ground zero up to a distance of 1000 mi. You'd need about 200 hitting Splyn at the same time if you rolled all 1s. Climatic effects would be well below that of Krakatoa, although you'd have Chernobyl style wastelands in parts of Atlantis. The CS is said to have about 700 such weapons and the ability to bring another 250 online each year, but it's not realistic to imagine they'd all detonate undetected and near enough to one another not to permit Splyn the ability to skip dimensions.
No one on Earth has an ICBM capable of delivering a heavier warhead accurately but you could presumably teleport an enormous warhead in along the leylines Splyn has to sit on. Even if it did work and the GM didn't let him escape or survive within creatures possessed by one of his fractured essences, the players would've been facing 1–6 Splug guests, the Aztec goddess Chiuacoatl (who has a crush on Splyn), and at least one of the 3 planets already dominated by the guy, not to mention the survivors on Atlantis. They'd also paint a target on their own back for annihilation by the CS, who themselves would only use their nukes defensively.
The actual in-universe answer to this question is the SDF-3's railgun,¹ whose damage is all the damage to anything within a mile or so of its exact target. It fires from outer space and no one has any idea what just happened or who was responsible.
¹ Full stats including floorplans in Robotech II: The Sentinels, pp. 150–9. Apparently Archive.org's copies of the old Antikevs site also have stats for the SDF‑1 and SDF‑4 online, with a similar ultimate death reflex cannons. The Antikevs adjust the stats for assaults on planetary objects, limiting its ability to just punch a hole straight through the Earth: 4d6×1,000,000 MD, with a radioactive crater 3d6×10 mi. long and 4d6×100 feet deep, with a shockwave wrecking the terrain for a further 1d6x10 mi.